So: set kernel to 25KHz, check for bios power savers and sit back and observe...
...Working almost all day with everything OK, then freezes in the middle of the job. I took a closer look at the event monitor, and saw that it reported system reboot without shut down at the time it actually freezes. The only useful thing that I can do (am doing now) is to upgrade to XP :-( and see if problem will disappear.
Will keep reporting.