I had two problems with the slate’s function after testing it a bit.
Number one was the CPU. I used the T5200 I had in my T4210 before. I got it because of some overclocking ideas that didn’t work out (it has a 533MHz FSB) and kept it because of its great undervolting capabilities and the fact that it is a Core 2 and hence a good bit faster per clock cycle than an ordinary Core Duo. For some reason the TC4400 used a 666MHz FSB and hence the multiplier was locked down to 6.0. I couldn’t even change the voltage. The solution was simple and I just threw in the T2300E I also have. And it works a charm.
Number two was more complicated. The shielding of the digitizer wasn’t perfect and when playing around with it I noticed that under some circumstances I got a wildly clicking and moving cursor on the upper right edge of the screen. I can’t really say what was causing the interference. Neither CPU load alone nor intensive RAM operations did anything, but in combination this might have been it. The in-place FFT setting of Prime95 could halfway reliably produce the phenomenon. I spent a lot of time trying to find out where the interference was leaking in, but to no avail. The back of the display was completely covered in aluminum foil, even around borders and edges. So I just made a wild assumption and stacked aluminum foil and paper to see if it was just not enough shielding yet. That was the case. After placing several layers of addditional aluminum foil roughly above where the CPU, memory and northbridge sit, the problems were gone.
Some minutes ago I finished installing Windows 7 on the now also changed harddrive. Technically the system is now working absolutely fine. So I can now go and put the final layer of filler on and give this thing a paintjob.