GT5656 shuts down after boot menu flashes on start up. Help

Hello new guy here,

Have owned my GT5656 since new and never modified it. Recently had prolonged start up sequence with the system occassionally turning off after the Gateway screen flash and boot menu. Gateway tech advised to reinstall Windows Vista. (still same problem) and then later use system recovery disk to repair/reset.(still same problem) This period lasted about 1 week and now the machine turns off after the Gateway boot menu.(no longer enters windows)

Installed a new UPC, a 500W system. System powers on and can hear the fan but same issue as before in that it crashes and turns off entirely after the Gateway Boot Menu.

What else should I try? video card or motherboard? Some other?

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Thanks for replying.
Inside was free of dust bunnies.
CPU has a fan mounted atop the heatsink. Fan sturdy but did not remove to see if the heatsink was attached to the cpu.
Will check RAM,CMOS and remove fan to see if heatsink is attached CPU this evening.
Other steps in the troubleshoot before I take it to the shop?

NB: Also checked for defective capacitors.

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