iMac G5-Ambient- Unstable for Power Up

I have read many questions & answers on Fix It in order to hopefully find a solution to my problem, although my problem may differ slightly. At times the iMac G5 will power up & function normally for 3-5 minutes & then freeze completely forcing me to shut down.It will power up normally at random mostly when it was unplugged for a period of time. Although at times it will chime to a blank Grey Screen or Apple Logo or Wheel or mostly no chime to a Black Screen. There is no overheating. The Capacitors look fine visually, so does the Inverter.

What I noticed 1 time it started up normally was the Time/Date was incorrect therefore I replaced the Pram Battery, no improvements. I could reset the Pram only once, 2 chimes but the screen remained Black afterwards. At times when I remove the cover, I get 3 LEDS on & at times only 2,this iMac resets the SMU automatically when unplugged for 10 seconds. Frustrated, running out of patience & ideas. I do not want to ruin the Hard Drive with all these Start Ups. Any comments & advice will be greatly appreciated, Thanks again, Robert