Make sure that it's nothing to do with this:

http://support.apple.com/kb/HT2390

I've already sent in my iMac G5 twice early this year and so far it's running OK. Mine wasn't even listed in the range of serial numbers, but they replaced the parts at no charge anyways. Apple is aware that this is affecting all iMac G5 models, not just the ones mentioned on the link.