Now my system is running fine since 10 days.
The current settings are:
Linux kernel / GRUB:
enable_mtrr_cleanup mtrr_spare_reg_nr=1 hpet=disable
Option "RegistryDwords" "PowerMizerEnable=0x1; PerfLevelSrc=0x2222; PowerMizerLevel=0x2; PowerMizerDefault=0x2; PowerMizerDefaultAC=0x2"
The MicroSoft IntelliMouse is still atached.
Although I'm happy that the system seems to be stable now, I'm not so happy about the disabled PowerMizer as it's limiting my graphics performance now. (And if I set it to maximum power the fan would blow my ear of, so that's also not an option as most of the time the limited performance is fine for me).
=> Please fix this strange bug
(And I'm sorry that I didn't find a way to reliably reproduce it - except to work on that machine for a few hours interactively)