You're the man!
Thank's a lot Emphroon that really helped us out.
Your suggestions work for us!!
We didn't not only kill one but three Mobos trying to downgrade the bios version. :-(
Then we swichted to Suse 8.2 which did not have that problem, but now we can install debian again.
Do you think that disabeling the IRQs will slow down system performance?