Dell Dimension 2400 won't turn on

Hi, newcomer so please forgive. I have dell dimension 2400 that won't turn on properly. When power plugged in and turned on system tries to start with green light on power button, fans going, lights quickly come on then go off on keyboard, then power goes off for 3 seconds, then tries to power back up. Nothing on monitor. Diagnostic lights at back are y,y,g,g. No beeps. I have opened case and reseated everything. Took out all cards, disconnected dvd drive and still the same. When I took off the heat shrink over processor, top of processor chip is chalky.
Please help.....
 
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