MAC G5 Freezes and Fans Go Nuts!

I have a MAC G5 that will freeze on startup or decide to freeze in the middle of something important.  When this happens, internal fan speed gradually goes to max and makes a lot of noise.  I have manually reset the SMU and PRAM and ran the disk utility to repair permissions.  Permission repair did fix the problem for a while.  This is a new OS 10.4 install.  My take is that something is failing and overheating making the fans do overtime.  Any ideas?  I think I am going to try a new hard disk next.
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I have had the same problems in past and these are the keys (some of which you have done)
- update OSX ( you said 10.4 and current rev is 10.4.9)
- update firmware
- reset pram
If you have done all these..
Did you buy all your RAM from Apple?
If you bought your computer from a 3rd party, chances are they offered an extra 1GB ram deal. While mixed brands COULD POTENTIALLY be a problem, more likely it is the RAM itself. So open up and remove the RAM to inspect and see what you have. 2 Apple Ram chips or something else?
Remove the non-apple memory and use your computer. if you cannot reproduce the error again, it's your RAM.  Most offer lifetime guarantee, so you'll have to check with mfr on that.

 If you can reproduce your error again, even after all of the above, then it may be the logic board or other hardware, but the vast majority of problems are fixed by the above. I've experienced the issue on (2) computers but the solution was different for each (one memory, one software)
Eoin OSullivanConsultantCommented:
The Hard Drive in not likely to be the problem ,.. more like a memory leak or cpu intensive process that is hanging/crashing

When the problem starts.. open the Activity Monitor application .. in your Utilities folder

Look for a process that is using the CPU ,.. possibly a crashed or leaking memory application.
Application Monitor will allow you to KILL the offending process
The fan should return to normal

Can you let us know what process is using the majority of the CPU when the FANs go into overdrive?
Try booting from the OS Install disk and select Disk Utility from the menu. Then run Disk First Aid to repair the disk. You could also download and install

If Disk First Aid and SmartReporter give your disk a clean bill of health, then your problem is not likely drive related.
What version of 10.4 are you running. Is it updated all the way through 10.4.10  The fact that it is freezing on boot as well makes me think more about core kernal issues than a memory leaking app though both could be in play  for crashes after a succesful boot. The ramping up fans is like being halfway to a kernal panic. Any time I have ever kernal panicked I always get the full bore roaring fans.  I would download the manual update reinstall for PPC after repairing permissions again. Also get a copy of Disk Warrrior 4.
It will fix more deep drive errors than any of my other utiltities.
This was happening to a machine we had, the only way to make it boot was resetting pram and nvram.
I actually put another hard drive in and did a fresh install on that and havent had the problem since so your original idea of a new hd might work.
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