I find OCCT GPU to be better in measuring stability in real world gaming applications. Furmark, Kombustor etc. often reports a false/positive situation.

My GTX480 @ 950/2200 ran through furmark, kombustor without any problem - although was not stable in Metro, BFBC2, Dirt 2, Assassins Creed 2.
By using OCCT GPU, I found the rock stable clock in all benchies, games etc. to be 900/2050 I had "only" 34 errors, during a 20 min test.