My first thought is a bad power supply. Check the voltages in the BIOS if you can.
If they're under the nominal by more than 10-15% I would change out the PS.
Even without bad voltages you could have a sudden power drop which would cause the system to reboot or become unstable.
Also, could try Stress Linux. Linux distro for testing systems under heavy load.
Worth a shot and might be revealing. Google for latest version.
Disabled auto restart on failure, crash happened, no log or BSOD.
Will try stress test Linux
I almost want to say with 100% certainty its the PSU, do you have another psu to test with?
Install any new hardware recently?
Stress testing most defiantly can help with that. However if a stress test is put on a bad psu, that might cause bad things to happen. Especially with a very weak (not high quality) psu. I'm not saying his is. But in most circumstances the psu will save itself. Ether way we will figure out the culprit, but I think a different psu test should have been done first. But that is my own opinion. (Yours is still a very good suggestion) Different methods. For stress testing I would use prime95. But based on his problem your method for stress testing is better then prime 95.
I've seen some really under powered PS in computers. Some are ridiculously under powered too.
I have a neighbor who bought one of the eMachines budget line with AMD Sempron CPU (socket 939 I think).
They're not computer savvy and probably got it for Xmas or Black Friday.
Probably 3 years ago now. They asked me to have a look when it wouldn't power on.
One of the caps was charred. It was only a 275W though.
I put in a 350W I had laying around and it killed that one too within a few days.
I finally got them to buy a 500W for about $40 and that PC has been fine ever since.
Has Vista Basic(!), but hardware will likely last a while more.
They had no symptoms of failing PS though. It just died.
Might have been a power surge though. The caps were really charred.
Check all ATX power connections on the MB and PSU. I've seen similar issues that traced back to a bad cable/connector.
The PSU is a BeQuiet Straightpower L8 730w (too big yes but it was £60).
I myself am pretty sure it's the PSU.
Your PSU might not be bad, but the 12V supply rails are underpowered. The specs show 12V@30A x2 So 60A combined, but I think that might be the problem. 30A on one supply rail might not be enough to keep a system happy under heavy load.
Get a PSU with one 12V supply in the >40A or 50A on one circuit. Split rails might not be as good.
Mine is a seasonic ss-650km. Cost was $100. I'm not sure if you can get one in UK but it has 54A all on one rail and can handle a Haswell or so the box says.
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