Core i7 920 @ 4.0 Ghz in Prime95

Generally people run Prime95 at least overnight to determine if a system is stable. However, if your load temps are around 90C, you SHOULD NOT run prime 95 for more than a couple minutes until you get them in check. The i7s should not exceed 70C to be safe.

Your heatsink is not the greatest, but I don't think it's that bad either. Do you have an unusually hot room? Bad case airflow? Did you perhaps put on too much or too little thermal compound when you mounted the heatsink? Is the fan on the heatsink running ok? And what CPU core voltage are you using on your overclock?

Also, what program are you using to check your temperatures?

That's a pretty high voltage, so it could just be how your heatsink performs at those settings. You might consider reseating the heatsink if you would like to keep your settings, but I would say you are running too hot to stay there without some sort of change.

So should i try lowering the voltage? I just downclocked to 3.5, idling at like 55c now.

got it at 3.5ghz , 1.066v, bout to run prime 95..

That's fine - you could even push it a bit higher if you want. As long as full load is under 75-80 (and I mean that it doesn't exceed 80 even if you run Prime95 largeFFT for 8 hours), it's fine.