G4 just replaced power supply , it powers up , but the screen is not on and the mouse and keyboard aren't on either. A few times the computer came on with screen working but then it froze up, can't do any type of reset cause of keyboard not working. Help!

Hopefully jmac has some advice on that, I only have guesses.

Pull all but one DIMM and try testing again.

If successful, add a DIMM and test again.

And so on......

If it keeps crashing, it doesn't necessarily mean that the RAM is bad.

Rember rums in the OS, so it isn't quite the same as running in Single User as you can with Memtest.

The crashes will leave results as "inconclusive" though doubt about the RAM will remain.

Is this a MDD G4

If so, download and burn the AHT to disc and run those tests.

If not a MDD, the AHT is available for QS, DA and GE machines:

Apple Hardware Test Images and Information

Run the AHT first.

If the CPU, logic board, graphics card, etc. pass, then do the single DIMM memory test.