RichL , i really dont think its your xbox mate. do you not think its strange that it only happens on PDZ, mine does the same. so do plenty of others. you also get warning lights appearing on the XBOX360 before the CPU overheats enough to shut down, so if you havent got them then its not that. Also, if the CPU HSF was poorly fitted, and bad contact was an issue, it would once again effect everygame, all of the time.
to Summarise - dont go sending back your xbox360, its fine ( unless you are getting warning lights and crashes on all games, PGR3 doesnt count, as there is a crash issue when accesing dashboard at certain times that is being addressed. ) The problem is PDZ, its a poorly coded, rushed out, bug ridden game. sell it to some poor bugger 2nd hand