iMac G5 Help?

Ok so I am having a very weird problem with a friends iMac G5. He came over asking me to take a look at it because it wasnt booting up. The Hard drive would start spinning and the indicator light on the front would come on but nothing else.

I thought maybe a bad power supply or something wrong with the logic board. So i took off the back and ran a few tests. I took out the ram, the little battery on the logic board and let it sit for awhile then I replaced them and on boot did a PRAM reset with the keyboard, and it booted up. I was thinking ok maybe it was just a glitch or something. So i put the computer's back on and connected the keyboard and booted once again. This time the same thing happened as originally happened. just a spinning HD and the indicator light coming on.

This time I took it apart and did a SMC reset on the Logic board. turned it on and it started up again. I even logged into the profile and connected to WIFI. everything, left it on for awhile (~10 minutes) everything was still working, even took out the little battery, the ram and disconnected it from power for about 20 minutes. put everything back except the back of the iMac and everything again worked, so i thought again maybe a glitch so i reinstalled the back of the iMac booted it up and nothing, still doing the same thing as before. took the back off again and it worked fine.

Why is it doing this? has anyone ever had this happen to them? Any and all insight it greatly appreciated. Anything else I should try?

I also followed these steps and the indicators say everything is fine, but I can not check them while the back is on and it is having this problem.
(http://support.apple.com/kb/HT2173)

 

Thank you. I didn't even know there were powerpc forums lol yea I know they have a lot of problems, this one I dont think is the ram because its booting every time when the back is off, but not booting at all when the back is on.