Linx is a good program to use. It will usually give you an error before prime95 will. It's good for quick testing. It also makes your cpu 6-7C hotter than prime95.
I've had my i5 750 @ 4ghz for two months or so. Today the temp in my apartment was 90F with no AC
, and my idle temps went from 35-40C to 50C
Had to lower my OC to 3.8ghz for now.