What do you have the voltages set at? You should need a minor voltage increase with that chip if it is a G0 stepping chip. Although you need to remember that every chip is different. If Prime fails then 99% of the time it is a voltage issue unless you have the heatsink properly seated which judging by your temps you do.
Also did you try memtest 86 to test your memory because prime will do blended tests and if your memory is failing then that can be a problem. What about your other settings? Are you autoclocking your memory? If you set that too high it can crash out as well.
There are many parameters you need to set. Gigabyte boards make it way easy though.
I put it to 3.0 on all auto voltages and ran prime95 for a couple hours. It got pretty hot, near 70, but it will because your voltages are high. I didnt fail though. So i went back in and saw the auto voltage was 1.40 (could vary on your board). I manually set the voltage down a few notches and booted up and primed it again. Just keep doing that until it fails. Then when it does, go back in and move it back up a notch until it works.
Thats basically the best I can tell ya, mine works just fine and it idles around 40 and gets in the low 50s when gaming.