I HAVE A SERIOUSLY PROBLEM WITH MY GIGABYTE GTX 560 NON TI!
WHEN I PLAY GAMES, ABOUT AN HOUR LATER MY GPU USAGE GOES 0-10% AND MY COU USAGE GOES TO 100% AND I PLAY WITH 1 FPS FOR 10 SECS!
70->75 C is not that abnormal for Core temps with the CPU loaded and when using the "poor" Stock HSF. His problem was more indictive of a driver problem, which is what it turned out to be.
If you have not done, it is a good Idea to verify:
Temps and voltages (even thoe not the problem)
.. Cpu, run prime 95. This will also test Memory and CPU for stability while checking effectiveness of HSF. Since you have the stock HSF that may limit your run time - DO NOT go above 85 C for individual cores, Thermal throttling occurs around 95 C to protect CPU. About 15 Minutes is althat is need to check Temps and voltages (primarily your +12 V rail), But for stability need to run a min of 4 Hours, and Your HSF may not support so may need to put that off until after new HSF - MUST run prime 95 to verify stability when you start to OC.
.. GPU I run furmark and monitor GPU temp and again the +12 V rail.