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System randomly rebooting

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November 23, 2009 6:45:29 PM

My computer is randomly rebooting. I've tried everything I can think of to diagnose the problem and have so far come up empty. It's a QX6850 on an EVGA 680i-A1 motherboard. The reboot is truly random; it can happen a minute after boot or not for over a day. The frequency of rebooting does seem to be increasing. The problem was occurring with mess frequency under Windows Vista- I am not running Windows 7 Pro. There is no blue screen on reboot, and there are no entries in the Event Viewer. On reboot, USB devices freeze, and are not usable again until there's either a full power cycle or the OS starts again.

Here's what I have tried;

* Ran with no start up programs/minimum drivers
* Checked CPU and VGA temperatures, all are in acceptable ranges
* Ran Memtest+ for five cycles, then tried half memory in/half out just to be absolutely sure
* Removed all unnecessary devices (sound card and front panel USB/1394 connections)
* Replaced the video card
* Replaced the power supply
* Replaced the motherboard
* Updated BIOS
* Underclocked the CPU
* Increased memory voltage .1V

I'm officially flummoxed. Any tips would be greatly appreciated.
November 23, 2009 6:51:02 PM

I should've read the post before posting... The problem was occurring with LESS frequency under Windows Vista- I am NOW running Windows 7 Pro.
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a b B Homebuilt system
November 23, 2009 7:54:09 PM

Have you tried moving the computer to another room (or better another building). Its possible power brownouts could do something like that.
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November 23, 2009 8:01:30 PM

I've considered that. The whole thing is plugged into a massive Ultra UPS, so my thought was that should prevent a brownout problem. I have tried bypassing the UPS, but the problem persisted. I have no way to test the PC in another building.
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November 23, 2009 8:17:30 PM

is your psu dying?
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November 23, 2009 8:24:26 PM

I did replace the power supply, as mentioned in the original post.
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a b B Homebuilt system
November 23, 2009 8:36:04 PM

What model PSU and GPU?
Did you turn off all "green" power saver features in both windows and the bios?
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November 23, 2009 9:21:43 PM

PSU: Cooler Master UPC 1100W
GPU: BFG nVidia GTX 275

All power saving options are disabled except for turning the monitor off after 20 minutes of inactivity.

As I mentioned the the OP, I've replaced both of these. Originally it had an 8800GTX and an OCZ 1000W PSU. The reboot has occurred under all configurations.
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November 24, 2009 2:03:39 AM

Have you tried running CPU stress tests (e.g. Prime95)? I see that you've checked your memory via memtest but what about the stability of the CPU itself?
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November 24, 2009 1:19:08 PM

I know you mentioned that you replaced the PSU but I am pretty certain that it's a PSU fault, especially if you tried with a different motherboard...

Does it occur more often when playing a game or generally when the graphics card is consuming more power? I had the same problems with random reboots and no blue screens and I've invested in a corsair PSU - no problems so far...

Don't spend your money on a PSU yet but give us more details (i.e. when does it occur? is it random or more frequent when gaming? etc).
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November 24, 2009 7:44:25 PM

I am now testing the CPU.

The chances of it being the PSU are nil, unless you're suggesting that it's the exact same problem with two completely different PSUs. There is no difference in the problem from one PSU to the other.

The problem is completely random. It does not occur in regular relation to any activity on the computer.

Update: I can't run Prime95 for long enough to detect any problems. The computer crashes before it can get anything measurable done.
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a b B Homebuilt system
November 24, 2009 8:19:45 PM

I believe that is called "a clue". ;) 
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November 24, 2009 10:26:23 PM

Maybe, but the computer reboots randomly anyway. The fact that it reboots while doing a CPU test... Well, it's rebooted while doing anything and everything else. The chances that I can successfully run Prime95 long enough to get anything conclusive out of it is nil, even if the processor is fine.
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November 24, 2009 10:51:28 PM

An unstable system will probably reboot randomly. Have you tried resetting everything into their stock settings? Unless your mems aren't really stable enough at stock (i.e. some ocz mems that would need a small jump in voltage to be stable)
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November 24, 2009 11:57:33 PM

Yes, I've tried that. I've also cleared CMOS and updated the BIOS.
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a b B Homebuilt system
November 25, 2009 12:07:11 AM

The only things left are to replace the CPU or move to a different building to remove environmental issues as the cause.
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November 25, 2009 12:27:19 AM

If I had to guess at this point, I'd say it's the CPU. The UPS should be preventing any power issues. Unfortunately, I have no way of testing to see it's the CPU without getting another CPU. Suck.
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November 28, 2009 5:59:11 AM

Ive had a very similar problem, check BIOS and make sure the cpu and system temperatures at which the machine will restart itself are not lower then necessary
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