Help diagnosing spinning ball / crashes

Hi - looking for help to diagnose a problem with my iMac G5 2.0 GHz 17" (MacOs 10.4.11).

Symptoms are that after a few hours running, the spinning ball appears more & more often till eventually locks up, often with the fan(s) going into overdrive at the same time. Sometimes prior to the lock up the hard drive starts clicking. Once it hits this point the only way to recover is to force power down & restart. Often have to leave the machine alone for a few hours before it will restart successfully.

My suspicion is that it may be the hard drive over-heating or possibly about to fail. I've checked the SMART diagnostics via Disk Utility and Disk Warrior and they both say everything's fine, except once on Disk Warrior when SMART check came back with: "The hard drive's built-in temperature sensor indicates an above normal temperature of 150.8 F (66 C), this might indicate a failure or pending failure that could result in loss of data". I checked again immediately after and it reverted to OK again.

Btw, I had a logic board failure some 2-3 years back. This was replaced by Apple under the extended warranty. I've checked the caps now - they are all pristine. I've also cleaned out all the dust to give the fans the best chance to work.

I've used Hardware Monitor (http://www.bresink.com/osx/HardwareMonit...) to log the temps. This reports three temp sensors: CPU, "Hard Drive" and "SMART Disk Maxtor 7Y250M0 (Y668LJSE)". The last two seem to track each other with "Hard Drive" generally 7-9 degrees lower than "SMART Disk", so I guess one may be a sensor outside the drive and the other on the inside.

Immediately prior to the last failure, the "SMART Disk" sensor had hit 68 degrees C, which was a max since it had been started (although it had hit this temp a couple of times before without locking up). It locked up for 5 mins and when I restarted the iMac, the reading was 73 degrees C.

I've tried adjusting the AppleFan.kext to kick the fans in earlier to keep everything cool (as per http://bitsandpieces.info/MDDFanExtensio...) - going for a 10 degree reduction to default - but it's not obvious that this is doing anything.

So - can anyone give me advice on diagnosis / next steps. Does this sound like a fan problem, a hard disk problem, or something else? And how do I remedy it - should I replace one / all of the fans, replace the hard drive, find some utility to get the fans to run faster (haven't located one yet apart from the kext mod which does not appear to work), or ...?

Any suggestions / advice very welcome.

R.