PC doesn't always boot and restarts when under load

Samp21

Bit of a wierd problem here, sometimes my PC won't fully power on. What I mean is that sometimes when I press the power button, I hear a fan whirl up and everything just stops, the power light stays on but everything is dead, the psu, the harddrives, the fans, everything, and the only way to get the PC to start is to hold the power button in for for a few seconds until the power light goes off cross my fingers and try it again (the reset button won't work). And sometimes when I'm doing something like rendering video or something which requires the PC's undivided attention, either the same thing happens or it reboots with nothing in the event viewer. My PC is a couple of years old and I do plan on upgrading but I would like to fix this problem, as I would like to sell my PC when I get a new one.

Here's my PC's spec:

350 Watt PSU
AMD Athlon XP 3200+ Barton
K7VT6 Motherboard
1280MB Ram
Windows XP SP2

joePC

Check you CPU temps and the stand offs on the motherboard, make sure there is nothing touching the back of your motherboard.

Could also be your PSU.

Naikon

Yeah, try the stand offs and ensure they aren't touching any bare metal causing a short circuit.
I had the same symptoms after a new build and I solved the problem by replacing the stand offs.

5uspect

Check the heatsink for fouling. Sometimes they can get full of dust and overheat.

Anti

Sounds like a dodgy powersupply to me. Replace it with a spare if you have one

SeanW

Could also be bad RAM - I find that inexplicable instability often has a dodgy module of RAM as the cause.

extra-ordinary_

Unexpected reboots often points towards the PSU.

Samp21

Thanks to everyone who replyed, big help, ended up being the PSU