xXbunnyXx

Use monitoring software during gameplay to check if it's really overheating.

what board do you have? there should be two pins on the board labled CMOS CLR or something like that. Jumper those two pins with a screwdriver/something metal and it resets it. 
 
As to cause.... could be any number of things, board problem (check with different GPU in your current board), BIOS, faulty power connects, ect. 

That could be it.
 
I would start by installing and checking all connections, esp power and if modular PSU check PSU side connection too.
 
Does your PC pick it up when installed but your outputting video through the motherboard?

Double post sorry

You need to try your graphics card on a different system to make sure.