You're getting 10-15 minutes of uptime before the GPU overheats? Are you absolutely positive the fan on it is running?
If not, you may want to check the voltage in the BIOS. The voltage may have been inadvertently stepped up (you can check the proper voltage settings online, probably from the vendor website even). Another issue with the voltage may be region, some regions use 110AC, others 120AC, and so on. Of course, naturally, the PSU is supposed to regulate these things.
Finally, heat rises. Do you have any cabling hanging loosely above your GPU? If so, try and bind it. Always try to run cables behind the motherboard tray if at all possible. Always bind cables as much as possible. Maximum airflow is essential to proper cooling.
Inevitably, if the cooling issues persist, then I suggest you venture into the world of case modding. My current workstation rig runs a refrigerator compressor to provide more than adequate cooling. Of course, it would probably be much easier for you to install a liquid cooling system.