PC locking up...

I'm working on a PC that is locking up. After 60-90min of use, the PC locks up and I have to hit reset to restart. Then it's good for another 60-90min. The temps are good (max of 42 deg C) and I ran a HD test (OK) and a RAM diagnostic (OK).... chkdsk found and repaired errors but still the lock ups. And the weird part is that after 2 or 3 lock ups, and hitting reset to reboot, the HD boot priority changes in the BIOS and the PC tries to boot from SATA port 2 instead of SATA port 1; therefore no Windows! Specs are as follows: Pentium 4 s775 3.0ghz, Motherboard: ASUS P5ND2-SLI, 1gb RAM, 400w ATX 24pin PSU, Radeon x1600pro 512mb PCIX, 2 x 250gb Hitcahi Hard Drives, LiteON DVDRW, ASUS DVDRW, no floppy. I checked the ASUS web site and there is no mention of this problem.... The lock ups are one thing, but the BIOS changing by itself is very odd. And the only way to change the boot order back is to press F5 in BIOS to load default settings. The BIOS won't let me manually change the boot order of the hard drives when drive 2 is set for the first boot device.... Any help or suggestions will be much appreciated!!

hopperdave2000

Oh yeah- Running Windows XP Pro SP2 w/ all current updates.....