I can run Intel Burn In Test and Prime95 from 12 hours 24 hours WHATEVER. And I'm STILL not stable when playing a regular Steam Game and I crash and burn. What I do is run IBT WHILE playing a demanding game. Now THAT, will tell you if your FULLY stable. That's my way. And personally I can't see doing it any differently IF you really want to be stable. Who want to be on your PC knowing that at any point in time doing something really demanding may crash your PC?
+1. Gotta really stress out your cpu. Intel burn test, plus prime 95 and play a game all at once even though your gonna get horrible fps, it will push your cpu to its limits.