Hard Shutdowns for Unknown Reason?

old desktop. hp d330 board, 3.0ghz hyperthreading p4 northwood, 1024mb pc3200 ram, geforce 7600gs, sata hdd, ide dvdr/w, running ubuntu 12.04.

Not a lot of power draw, but it's been sporadically hitting hard shutdowns/resets. temperature sensor logs show that cpu, gpu and chassis are all in normal ranges. HDD temp is sometimes a bit low but not critically. syslog records nothing relevant to the shutdown and nothing out of the ordinary, and there is no patter to where it cuts off, suggesting hardware origin.

Memtest86 reported a few errors but after reseating the modules they have since passed all of many complete tests error-free.

The resets have happened using two different psu's, both rated well above the pc's power requirements. Voltages are well within the margin of 5 percent of normal, generally 2 percent. I don't have access to temp sensors for them but they're never warm to the touch. Unless they are maintaining constant voltage but losing power and thus current, tripping their undercurrent limiters, I doubt they're at fault - particularly since that would mean they both went bad at once.

So - not software, not heat, not bad ram, and probably not the power supply. Could it be a damaged mobo? I've never seen a bad mobo that worked perfectly other than resetting itself, as this one does, for example right now posting this. Something about the cpu design (one of the first "virtual dual" cores)? Ideas?