PC rebooting (semi-)randomly

I built a PC with the following specs for my dad last Christmas.

Motherboard: Gigabyte GA-EX58-UD3R

Processor: i7 920 2.66GHz Socket 1366

Memory: 3GB (3x1GB) Corsair XMS3 DDR3 PC3-10666

Graphics: ATI Radeon HD 3870

Power: 650W Be Quiet Dark Power PRO BN073

OS: Windows 7 (previously 64-bit, now 32-bit)

Now it has started rebooting semi-randomly (see the point about stress testing below). If it reboots once, it will usually reboot again and again, within seconds or minutes, until it has a rest. Windows doesn’t log an error when it happens; it just logs the fact that it has rebooted unexpectedly, after it has rebooted.

It worked fine for about 9 months with exactly this hardware. It isn’t (and never has been) overclocked. The BIOS is on default/recommended values for everything non-trivial (e.g., USB keyboard detection has been switched on, but not much else).

Neither the CPU nor GPU are overheating.

We’ve tried removing each of the three RAM chips, and running with just two. It makes no difference.

We’ve tried a clean re-install of Windows 7 (both 32-bit and 64-bit). We tried installing onto a different hard disk.

It survives several hours of prime95 stress testing (large FFTs is all we’ve tried so far), but it reboots within a second or so of stopping the test!

Stress testing the GPU appears to have no effect.

Does anyone have an idea of what the problem may be? Thanks in advance.
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  1. That's an interesting problem.

    I can theorize I guess.

    1) The board could be making voltage adjustments under load that it is not making at idle. The adjustments could be to the CPU or RAM. As a result the system becomes more stable under load. This would point to either MB hardware or incorrect default BIOS settings.

    -The first possible solution to this is the easiest and cheapest. Update the BIOS.
    -The second test would be to use only one stick of RAM. This would lower idle voltage demands...another easy attempt.
    Along this same vein you would run CPU-Z and make sure the settings for the RAM in the BIOS match what you find in the SPD tab. Take any RAM settings off auto and enter them manually where you can.
    -If the actual voltage regulation of the board is messed up you will need to RMA it. That would be the last thing to try on my list.

    2) Bad PSU. Power supplies must make adjustments for different loads and yours might be fine at say 120W, while at 80W it is struggling. Not unusual actually, although usually they just get very inefficient. This could be influenced by the temp of the PSU, which is then dependent on the temp of the room. A cooler room would result in fewer reboots.

    -Be Quiet does not make PSUs. Like most companies, they buy from the makers. As I recall that unit is ATNG... heh nope. It's FSP, and not one of their better offerings. That's going to be a prime suspect.

    I doubt the 12V rails are really separate, but it looks like there is a dedicated rail for the motherboard. That might explain why the GPU stress is not impacting stability. It's only the 12V1 rail that is causing issues. That's the rail attached to the MB.... but again, I don't know the actual architecture of that PSU. I will call in an expert ;)
  2. Thanks for the ideas, Proximon. I'll try them out and post the results.
  3. A simple way to check if a "multi-rail" PSU is truly a multi-rail PSU is to disconnect it completely from the wall and the computer. Then use a DMM to measure the resistance between each yellow wire and every other yellow wire in both directions (polarity matters).

    If the readings are all low and similar, you really have a single rail PSU.
  4. That makes a lot of sense. I would be fairly confused trying to puzzle it out from the internals I think. Some time soon I'll have to go find some old ones and break them down. Have to finish my basic electricity reading first.
  5. I decided to just buy a new PSU: a Corsair HX850. So far (after two stress tests and two days of normal use), it seems to have fixed the problem. I will confirm in a few days.
  6. ^ Well to recommend a cheaper but still good PSU for you to save 20$ is the XFX 850. Same 80+ certified and modular just the same. Plus has great reviews and it's cheaper.


    Reviews: http://www.hardocp.com/article/2009/12/10/xfx_black_edition_850w_power_supply_review/9
  7. aznshinobi said:
    ^ Well to recommend a cheaper but still good PSU for you to save 20$ is the XFX 850. Same 80+ certified and modular just the same. Plus has great reviews and it's cheaper.

    Thanks for the suggestion, but it's been bought and installed already.
  8. Awww ok then.
  9. Still no problems since the PSU swap. Thanks again to Proximon for helping identify the problem as the PSU.
  10. That's great news. The stranger the PC problem the more likely it's the PSU. It's a maxim.
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