I got a new MSI Radeon 7870 HD and have underclocked the memory to 750 and upped core to 1200.

Getting 390M at standard clock and 430M with above settings. Problem is regardless of settings, temp runs up to 80C, runs for 10-20minutes then GPU shuts down and hash drops to 400k.

So the newb question is my PSU has two 6 pin connectors Y'd on two separate cables - so total of 4. I'm using one Y'd pair for both required connections on the card. Could this be an issue?  Should I run one 6 pin connector from each cable? 

Feel like I've covered most other possibilities. Any thoughts are much appreciated.