I think my problem has been solved, for now anyway, but I am really hoping this might be it.
Daskeyboards have so far decided to ignore my ticket on this issue on my 16 week old keyboard, which forced me to go and hunt for new keycaps to replace the four ones with damaged stems - either that or do without my keyboard. So the new vortex caps arrived today and I quickly stuck them on the Das. The stock Das keys come off quite easily, but the new vortex ones certainly don't. When they're on, they're on, so much so that my little plastic ring pull isn't really up to the job. When the new caps were pressed down into position I immediately noticed a fairly solid click, as the little + slotted into place. The Das ones never did this. On pressing the new key I noticed an immediate change in feel and sound. Before the browns always felt a little grainy and I could never really fell the bump, it was more like a grainy hump. But now its much smoother and the bump is far more of a bump. The sound has got more of a solid sound, and the sort of springy, tinny sound has gone completely. To be fair I never really noticed until I compared the keys pressing side by side.
But the best thing is, apart from my z key, which I think might actually have some dirt or ash in it, the chatter appears to have stopped. In fact the z doesn't chatter, it just misses occasionally. So, my new theory is that the loose fitting, thin and brittle keycaps were the cause of my chatter on the Das at least. The Corsair was always different - cleaning never helped and the chatter spread across the board from left to right, it was also much more pronounced, and once it started it just got progressively worse until the board was unusable. The das behaved inconsistently - sometimes chattering, sometimes not. I'm hoping this isn't just another sometimes not.
So when I look closely at the stems on the bottom of the caps, you can clearly see the difference in width on the little plus slots. The Das ones are much less well defined and definitely larger/looser.
So I think that the Das keycaps are so piss poor that they have just gotten loose and brittle over time, and where causing an uneven strike of the keys. I had always been slightly aware of a bit of an uneven look to the rows on the Das, and I thought this was just a standard tolerance for the switch type as the Corsair was never perfectly straight either, but that definitely isn't the case because the new key caps line up perfectly.
I have to use the kb for a few days to confirm it is really gone, and give that z key a little lube or clean with something - I'm sure there are guides for that about. But I'm more confident than ever before as there is something about the responsiveness that I haven't felt in a while. I have to say I am appalled at the crappy quality of the Das keycaps, especially now I have seen something decent - its not cheap keyboard by any stretch. The new caps aren't perfect, you can see some stem marks on the space and right shift, and they sent me two 'D's and no hash key and they are very grey. But other than that this could turn out to be the best buy I have made all year.