Sounds like you tried to find the maximum core speed instead of doing what I said and scan for artifacts?
Scan for artifacts then watch the temperature to see how high it goes. What graphics card do you have? Most cards should be good for 90C and higher, although lower is always better. My 9800GT with the stock cooler ran in the mid 80's before I put a 3rd party cooler on it. It now runs mid 50's.