You need to increase the core voltage. There are a couple ways to do this with your board:
1.) You can modify the CPU VID (Voltage ID) pins so that the board detects a higher voltage
2.) You can use an adapter like the Lin-Lin ($8 at Compgeeks) to set the voltage using the adapter. The Lin-Lin is a Tualatin adapter for non-Tualatin boards, but also does a few other things, like allowing Coppermines on non-Coppermine boards and allowing voltage adjustments.
You shouldn't need more than 1.85v.
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