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I'm having a problem with my desktop whereby it sometimes resets itself semi-randomly. For example, I'll switch it on, it'll boot an OS and shortly after getting to the desktop it will immediately reset with no warning. The time isn't consistent - sometimes it does it before reaching login.

I'm pretty sure it's not an OS thing; have tried Ubuntu and a Windows install and both exhibit it. It also doesn't appear to be heat-related because sometimes it appears to be able to "get past" it and will then run stably even under load; if anything it seems to be worse from a cold start.

My gut feeling is some kind of power issue but I'm clutching at straws a little. Any suggestions on how I could go about testing it or trying to narrow the problem down would be appreciated.

The machine is four years old now so while I can replace components if needed, it's not worth enough that I'm comfortable buying new parts without being pretty confident that they'll fix the problem.

Thanks in advance for any help :)

Edit: Okay, the motherboard is a MSI K8N SLI; CPU is an Athlon64 X2 4200+. Has one video card, a GeForce 7800GT. 1GB RAM, not sure of brand; 3 hard drives, two SATA and one PATA. Flashed motherboard to latest BIOS some time ago.

Edit the Second: I thought I'd narrowed it down to the PSU for a while, but then it recurred again. I ended up pulling everything out but CPU, RAM and motherboard and it still seems to be stuffed (if anything, it's gotten worse in the last couple of days). I assume it's one of those three components, but the machine is old enough that I don't really want to spend money replacing any of them. So thanks for everyone's suggestions; much appreciated!

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Well maybe some info about your system components could be helpful e.g. power, board, model, etc. – Nobody Aug 12 '10 at 4:51
I agree with Ehsan, more info is required. For instance, I once had a machine that would leak memory due to a poor network card driver, and would shut down "randomly" as well when it ran out of RAM. On the other hand, it could be heat related, or your CPU heatsink is full of dust. – user3463 Aug 12 '10 at 5:09
@Randolph: Okay, but I have explained why I don't think it's OS or driver related, and it seems all backwards to be heat - one would expect that to be worse under heavy load, not from a cold start. I'm not expecting an immediate fix, just some advice on how I go about diagnosing it. – Peter Aug 12 '10 at 5:19
What PSU do you have? A power supply tester can test that, to eliminate it as a possibility. Also check capacitors like Spiff suggests. – user3463 Aug 12 '10 at 8:13

I would vote for running several passes of memtest on it. If you have ubuntu installed you can access a version of it from the GRUB boot menu.

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Good suggestion - at the moment it's not staying up long enough to run that, but would be worth a try if it did :) – Peter Aug 14 '10 at 5:48

Have you ruled out capacitor plague? It should be easy to visually inspect your aluminum electrolytic caps for venting.

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Had a quick look the other day but didn't spot anything out of order - will look more carefully though. thanks for the suggestion! – Peter Aug 12 '10 at 21:33

Check that the CPU isn't overheating.

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doesn't seem to be - BIOS suggests temperatures are pretty cool (unless the sensor's wrong, I guess?) - but thanks :) – Peter Aug 12 '10 at 21:34

The first thing I would check would be the graphics card, check whether it's clean and the fan is spinning okay if it has one.

If possible replace the graphics card with another to eliminate this (from your enquiries).

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