was given a 12" iBook G4 and an iMac G5 PPC 1.8 17" I think original version.

I've sorted out the problems with the iBook, but I can't do anything with the iMac.

It won't boot up every time - it either crashes with mental-speed fans and the spinning gearwheel, or when it does boot up within minutes I get the multi coloured spinner instead of the mouse arrow and the system locks, irrespective of booting from the hard drive or if I boot using a my installation discs. It's running Tiger, fully updated.

I can't do anything to assess what is wrong. i've tried memory that is known good and makes no difference. I've also done the ⌘⌥PR on startup, held the power button in for 5 seconds to reset things and still no different. The hard disc is known good as was new when for when i got the iMac.

before i rip the motherboard out and change the capacitors, is there anything else I can try? The capacitors do not look appear to have blown.