GPU Overheating ?

Hi all, hopefully someone will be able to give me a difinitive answer to the cause of my problem with my HDX9200. Recently it has started to show signs of possible over heating of the GPU?
The symptoms are random freezing becoming totally unresposive and haveing to hold power button to shutdown and then reboot. I removed the rear panel to check that the fans are not clogged and they were reasonably clean as I regularly clean them. I replaced the rear panel but left the screws out and when it next froze I popped of the rear panel to find that the GPU fan had stopped.
With the rear panel off I am able to reboot and use the Dragon without any issues ( more airflow) ? I am able complete all the tests and the play the game in 3D Mark 06 and the only way to make it lock up is to repeatedly play the game untill it locks up with a max temp of 60C with out the rear panel fitted. With the rear panel on it will freeze/shutdown straight away when I attempt to play the game with a max temp of 49-50C.

Possibilities? Is the GPU, CPU or Mother Board faulty and if so, will baking or a reflow cure the problem? Is the GPU Fan on its last legs or maybe the 130w non original PSU?
I am using Driver Version 8.17.12.5721 and using my Dragon/Gecko lol. to type this post with the rear panel removed in a warm open fired room and a GPU temp of 35C. All temps mentioned are checked with Speed Fan 4.49.

Any help or advise appreciated
Many thanks Mike