Its unlikely that cooling and memory errors that only
appear after long memtest runs have anything to do
with my problem:
I disagree here. I had some memory timing issues on my system, and they
would usually lead to problems in test #5 of the latest memtest86+.
They also lead to nearly exactly reproduceable kernel panics in or
around ntpdate.
But i
can keep it on all night with Knoppix with no problem.
So that should eliminate cooling.
Yes, that sounds right. But remember that Knoppix is 32bit, so you are comparing apples to oranges here.
My money would still be on the memory / memory controller. Try less
aggressive memory settings in the BIOS (or taking out one memory stick
if you use 2), that did help in my case.
The other lead to investigate would be the hard disk controller. Maybe
a more recent kernel would solve the problem? Or try installing without
a SATA disk connected.