i bought a new cpu and i booted it up and it was just fine. but after several few hours it went off and was on again for a few seconds then off and so on. i have it checked to computer repair shop and they just replace the cmos battery and everything is fine again, but after a couple of hours use it went off again. it wont boot up, just on and off repeatedly. please help.

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I suspect your Power Supply.
What kind of CPU did you upgrade to, and what is the power-supply wattage?

If you are using a fast CPU, it could be that the power-supply does not have the grunt to run it long-term, thus, the PSU slowly overheats over an hour or so, then the thermal protection kicks in, and the system shuts off without you asking it to.

Also, check my post(post #4) in this thread (http://www.pcguide.com/vb/showthread.php?t=61724) about heatsinks and thermal grease.

Did you use grease when you put the new CPU in?

Incorrectly greased and heatsinked CPU's will run hot, and could cause the system to repeatedly reboot.