PowerMac G5 1.6GHz SP Freezing



I have been using a PowerMac G5 1.6GHz Single processor for just over a year now, and have been noticing a problem in that it unexpectadly freezes - it just locks up completely - cursor does not move, anything playing in iTunes stops etc.

I have still been unable to find a solution to this problem - I have replaced all of the RAM and bought a new USB hub on advice from a technical support engineer. I have tried cache repairs and Disk Utility Repair Permissions and Repair Disk, but it still freezes. I have also run the Apple Hardware Test, and everthing passes okay. I have even taken it into a repair centre, but they said they were unable to recreate the problem, and now the guarantee has expired.

More recently, I have noticed that the freezes tend to occur more often in hot weather (possibly a cooling issue?), and after the machine has been running for a while, but as far as I can see, there is no relation between the freezing and high CPU usage.

Since the machine has been at the repair centre, it has developed a new symptom - the fans begin to spin very quickly after the crash has occured, much like they do when running a hardware test (should they even do this for hardware tests?).

Is there anything else I can do to try and solve the problem, short of buying a new computer? Any help is much appreciated.

Many thanks,