lockup 4.0.2 on dual Xeon HT Dell Precision 450 Workstation

boots fine; any single task runs fine.
running 2 programs often locks up solid - mouse freeze, power off.
e.g. 2 copies of clamscan or f-prot has always locked system within an hour, usually in 10min. (running in 2 root shells)
1 copy will run for days ok.
bootup shows 4 CPU's. (2 cpu x 2 HT)
i7505 chipset IIRC. Additional drives on Promise Ultra100Tx2 IDE card.
Disk I/O in WinXP is fine; 200G data rd/write w/o error.
memtest runs fine. (ECC RAM anyhow)
(in WinXP, 2 prime95's run fine for over a week)
ran mprime -t w. 4 jobs 13hrs ok (Knoppix)
no disk I/O involved w. mprime tho.