iMac G5 - blank Screen and High Speed Fan

I have ready similar posts, but nothing I have done will fix this problem. I have an original iMac G5 20" 512MB PowerPC OS X 10.4.11. It has been great for many years. Suddenly, for the past two months, if I leave it on and go away for 60 or more minutes I come back and the system is locked up (only holding down the power button for 5 seconds will shut it off) and the fan is running full speed.

I opened it up, went through the four light diagnostics, passed everything, but did the RESET anyway. BTW - the iMac is well ventilated and has no dust built up inside. It is still locking up after letting it sit unused for a long period of time. After more research I download the temperature monitor software. My CPU is running at 72 degree C. I don't know if that is too hot?

Any ideas for next steps? New power supply? Software patch? New power management card (is there one?)?

Thanks,
Mike1872

Same answer Apple Hardware Test and please don't say you do not have the original install discs.

Not following?

HarryB - Thanks for your response. What do you mean "same answer"? I opened the imac and looked at the four LED's on the back as I powered up and did some testing and a reset. I never used any disks? Are there diagnostic disks I should be using? Could you point me to the KB article?

Thanks,
Mike1872