Is it still on the original heatsink?
Go into the bios, then up the FSB by like 5, then boot into the OS, and run prime 95 for 10 mins. Make sure to watch your temps, if it hits 60 then cut it and drop down to the last number, just keep repeating untill you're where you want it or is getting too hot. But once you're where you want, run prime95 for a longer time (i think over-night is recomended if the temps ar low enough to not worry) to make sure it is staying stable.