Random crashes, loss of USB on reboot

Hi wonder if you can help.

My PC has some unusual problems; it has started to randomly crash (under load, ie gaming etc).  When it crashes it just switches off, no warning, no error message, no hang, just literally switches off.  I cannot switch it back on until i flick the power switch on the PSU off and then on again.  After the third crash the PC came back on, but would not recognise any USB devices(ie mouse, printer, removable storage); BUS powered devices still get power but it seems that the OS cannot see them. When returning to windows the is no "the system has recovered from a serious error etc, message.   Please help this is my main system with all my work etc on it, and i am really worried.

Hardware.
AMD x2 4400
2 gig corsair XMS
2x 36g raptor (boot)
3x320g WD (raid 3 array on XFX raid PCI card) Storage
Nvidia 7800gs GFXcard AGP
tagan 800W PSU
2x pioneer 109 DVDRW

CPU is at 50 C in the Bios, this machine is not and has never been overclocked.

Hope you can help and thanks in advance

Rob
robbie1974Asked:
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CallandorConnect With a Mentor Commented:
If it only happens during games, then it's either a bad video driver or a bad video card - try reinstalling the video driver first.
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CallandorCommented:
It may be a power problem, and power problems can be caused by either the power supply or motherboard.  Can you swap out the power supply?
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CarlosMMartinsCommented:
Download speedfan: http://www.almico.com/sfdownload.php
 and check your temperatures with it idling in windows, and after some minutes of 3D gaming.
It will also show you the temperature of your graphic card (which you can also check on nvidia's driver).

With all that hardware in there, it might be overheating.

Speedfan will also allow you to check the different voltages on your motherboard, so you can check if it there's any significant variations when you're gaming or having your system under load...

Other option is to search if your motherboard has any hidden "dynamic overclock" options.
I had the unfortunate experience with my MSI-K8N-Neo, that was indeed overclocking my Nvidia card and causing problems - and that option was hidden and you needed to press some ALT+Fn+Keys to be able to reach it.
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MnfCommented:
you have faulty power supply, try to replace it, or try to buy one with more power 350W and more is recommended
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robbie1974Author Commented:
thanks for your comments guys, let me provide a bit more info.

Psu has been swapped for antec Truepower 550w and back to the tagan 800w, no change.

However, my machine will happily run a virus scan, stream a movie and complete an overnight torture test on prime 95 simultaneously without problem.  However, any game seems to make it crash, the more demanding the game, the quicker the crash.

Also, after the crash my board seems to load "fail-safe" defaults which includes disabling the usb controller; so at least one part of the problem is solved.
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MnfCommented:
look at this maybe can help you
http://www.experts-exchange.com/Hardware/Q_22039143.html
seek the accepted answer
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robbie1974Author Commented:
thanks callandor, reinstalling video driver has worked.  Glad its something so simple; should have worked this out for myself but the loss of usb on reboot threw me there for a bit.

Thanks to all for their help.
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