iMac - G5 OSX Crashing with High Fan Speed

After running for a few minutes the system freezes and a message appears instructing the shutdown of the computer. A short while after this the fan speeds up to full noise.

One time the desktop was interrupted with the following white text in black block (unix style): -

system failure: cpu=0; code=0000001 (corrupt stack)
latest crash info for cpu 0:

(I didn't get to write the rest down).

In the few minutes after boot up I have tried running 'fix preferences' but there isn't enough time to complete before another crash.

There is approx 10gb free space.

Could somebody please give me an indication what to do next?

Is this likely to be software or hardware? Any help much appreciated.