Twitchy front end

I'd invest in a fit using the cube as the "base" bike.
solved several of my issues

If by twitchy you mean the wind catches the front wheel and steers it (as opposed to 'twitchy' in the fast reacting to handlebar inputs sense), then I'd say it's the bladed spokes and possibly the shape of the forks.

I had a Merckx EMX 1 with Shimano RS10 wheels (which have bladed spokes) and forks that have quite a large trailing edge at the top of the fork legs (for aerodynamics I guess) and I think it was the combination of those two things that made it a real handful in the wind. I would literally get blown from one side of the road to the other; I hated riding it in the wind. It felt like somebody jerking the handlebars. I tried a different front wheel and that definitely helped, but it was still a bit of a handful.

I now have a different bike, with Kinesis cross wheels with round profile spokes and less aero forks and it is absolutely fine in crosswinds, you get a gentle tug now and again, but none of the violent change of direction.

I agree about the position of the saddle looks quite forward. If you've shortened the stem already (which will make any twitchiness slightly worse) then my take is that you could have done with the 60 or 62 frame. It's not unusual for tall pros (Johan Van Summeren who is 6'5'') to ride a 58 or 60 frame with long seatpost and stem. JvS is a classics rider and Paris Roubaix winner so I doubt he'd have the most twitchy of set ups.

Looking at the picture of the bike, am I right in thinking that it has a zero lay-back seat-post? It's quite hard to tell from the pic, but it does look like it.

With the height you have the saddle, you are clearly long legged, and I would have thought you would want a fairly good amount of lay-back on the seat-post. You could counteract the increased stretch with a compact curve handlebar which would bring the hoods closer without reducing the stem any further and adding to the twitchiness.

You might want to consider going to someone like Enigma for a custom frame somewhere down the line. I had a couple built for very tall riders by them, and they were happy with the results, especially if you are long legged for your height.

@Rickh13
It is a really common problem, it seems sensible that making the cockpit shorter will make it all more upright and so more comfortable, but you have to have your position in relation to the pedals right first before looking at the other bits. You can always shorten the cockpit with compact bars and shorter stems etc.

DaSy, that's exactly how my bike was when I first got it - twitchy, numb hands after about 20-30mins on the hoods and couldn't ride on the drops as it was far too twitchy, felt like I was falling forwards all the time.

I moved the saddle back and also flipped the stem to bring the bars higher up this moved my c.o.g. back so more weight is on the saddle with only minimal weight on the bars. I also found the position of the brake levers on the bars were a bit low for me and meant I was getting uneven pressure on my palms, they now support my hands with even pressure across the palm when I put my weight on the hoods.

It feels like a different bike now, its far more stable, but is easily thrown about when I need it. I can sit on the drops for as long as I need (just need to lose the gut so I can maintain a tuck ). Riding at speed used to scare me, these days it's still edgy, but doesn't feel like a bucking bronco that is waiting for me to relax my guard before throwing me off.

Have you got the 64cm frame and not the 62cm, then? Geometry is here... http://www.cube.eu/uk/bikes/road-race/agree/cube-agree-gtc-race-carbonnw... With a 220mm head tube and a stack of 616mm the 64cm surely can't give you a huge saddle to bar drop, even at 6' 4"? I'm 6' 5" myself and the best fit for me would be on a 62cm.

The twitchiness is probably not just a function of the head angle as it doesn't actually change from the 56cm up, all at 73 degrees.

Read this it will give you an idea of why your bike is twitchy:

http://calfeedesign.com/tech-papers/geometry-of-bike-handling/

You should be able to compare you Cube and your Spesh.

Unfortunately ultra racy bikes that are suitable for racing, are often oversold to the general public, and these bikes tend to be twitchy.

You should be able to ride no hands. If I couldn't, and it were my bike, it would be on eBay. In the words of Chris Boardman "who wants a twitchy bike".

Good Luck

A common mistake I see in my workshop is that people trying to take the strain off their back due to a feeling of being too stretched out, tend to push the saddle forward on the rails to shorten the cockpit. This actually tends to make the situation worse, as you effectively move your point of balance forward in relation to the pedals, so you find yourself with more weight on the bars and having to push yourself back to stay in position on the saddle. This also manifests itself as getting numb hands or shoulders too.

The position of the saddle in relation to the bottom bracket will be the thing that allows you to take you hands off the bars without dropping forward, as, if the saddle is sufficiently behind the bb you can resist with your legs on the pedals. The further forward that saddle is the more your point of balance and so your inability to resist falling forwards increases.

This combined with a short stem may well account for the twitchy feel. I would look at the fore/aft position of the saddle first and accept that you may feel a little more stretched out, but this soon becomes familiar.

Try to tell yourself to relax, breathe, flex your fingers etc when descending/cornering etc, as the risk is that no matter what changes you make, the knock to your confidence makes you stiffen up, which will just exacerbate the twitchiness.

Difficult to advise without seeing you, but generally putting the saddle back will push your weight further back... accordingly you will need to shorten/raise the front end to compensate for the increase in stretch caused by moving the saddle back.

Is the bar width narrower than that of your spesh? When I up graded to a new road bike I found it 'twitchy' to start off with until I got use to it. It now feels really stable and I prefer it to my commute bike.
The others have give good options for things to change if it doesn't improve, however as jammy ray says, it might just be that it feels different from your spesh and it'll take a little time to get use to it.

Can you ride no handed in a low wind situation, or do you start to fall forward?

I'm trying to establish if you are carrying a lot of weight through the front wheel, which is making the bike feel skittish, or if its just the geometry.

The problem with big frames is the way they keep the front end tight, so the head angle gets steeper and steeper and accordingly, the handling gets faster and faster.

It may just be a case of getting used to it, or if you have got your balance too far forward, that can be addressed with some positional changes.

A cheap possible solution, you could fit a slightly wider tyre at the front or reduce the pressure by ~5psi, the slightly larger contact area might reduce the twitchiness. If you're on 23mm then going to 25mm shouldn't slow you too much.

J

What about your stem length? A short stem will make the steering quicker so if your stem is considerably shorter on your new bike this may have an effect. Other than that I would suggest that it is just down to geometry and the new bike having quicker steering by design.

Those flat Aksium spokes do catch the wind, try a different front wheel.

What wheels and bars, is it more wind, or geometry?