pc keeps crashing/freezing need help!

[mitzibishi]

i recently put together a media centre PC and the thing keeps crashing randomly, screen freeze. usually when the cpu is stressed for anything upto a minute or more, but sometimes it can run allnight and itll be ok.

when it does crash i cant turn it back on for awhile because it wont boot, and anything under half an hour it wont get past the boot screens and freezes again.

i just bought a new PSU (400w) because the one that came with the case was underpowered (270w), fitted it and everything was fine, stressed the CPU, playing games and doing simultanious virus scans and disk defragments, was fine all night.

today i turn it on and it crashes within 5 mins after i open firefox.

i cant figure out what it is, ive tried 2 PSU's, different sets of RAM, the motherboard in another case so it cant be the case at fault, the hard disk has been in another case so it cant be that.

could it be a software issue or is it either the CPU or motherboard that is faulty, anyone had probs with random crashes?

here are my PCs specs:


small media centre case with 1 case fan: even with 1 fan the case doesnt seem hot or even warm



Cpu: Q8200 90w with stock Q9550 fan, reaplied some decent thermal paste

Mobo: Asus Pegatron_IPN73-BA motherboard



Ram: 3gb of bog standard 800mhz (ram area seems warm)

GFX: 9500gt targa edition changed backplate to make low profile, tried with and without Gfx card a no joy

Hdd: 640gb 7200rpm

PSU: 400w

OS: Vista home Premium


I managed to get some speedfan Temp readings after i stressed the CPU for awhile and it crashed

CPU usage %:
Ave. 57%
Core 0 58
1 82
2 68
3 28
RAM used 52%

Temps:
GPU: 58C

System: 32C

CPU: 28C

CPU fan 1259-1278rpm

AUX: 8C

Core0: 65C

Core1: 62C

Core3: 64C

Core4: 65C

as you can see nothing drastic

i wait 10 mins and boot back up, go into BIOS and get temps, and it freezes in the BIOS, here are the readings:

CPU target 60C
CPU temp 19C
MB temp 25C
CPU fan speed 1318rpm

Voltages
Vcore 1.25v
Vcc 12v 12.30v
3.3v 3.29v
5.0v 5.06v


can anyone with any experience with this problem help me work out wether its a hardware or software problem, if it is hardware through trial and error im left with it being either the mobo or the cpu

[Mithrandir]

What make and model is the PSU?

You should Memtest your ram to see if they are stable or corrupted at your settings (might need some tweaking in the BIOS - could be the timings or need more juice, etc).