ATI tool crashing computer

[gla3dr]

I recently decided that I want to overclock my GPU. I tried using ATI tool to test how much I would be able to overclock it. When I start the test, it heats up my GPU, then at about 86°-87°, it crashes my computer. Why would it be doing this? Is there something I can do to fix this or should I try a different program? My GPU is listed in my sig.

 

[!!!Daniel!!!]

Not all GPU's can be overclocked mate, like you said, once it gets up to roughly that degree^^ your GPU will over heat and crash your computer