I am betting voltage is not the problem, I can bench Super Pi at 2.7GHZ with just under 2v with my Mobile 2500+ when the ambient temps I duct into it are low enough.
It's most likely your cooling, what kind of 100% load temps are you getting now?
Check out this thread to see how I got these temps in the pic below...
http://forums.pcapex.com/project_wor...p_freezer.html
Using anything more than 1.8-1.9v in general on XP's will result in load temps over 50c on mid to high end air cooling..generally watercooling is needed for 2v, and "Extreme" cooling ("Sweet air" Phase Change, Dry Ice or LN-2) is needed for 2.1v and above.
A few more "Tips" may be found in this thread
http://forums.pcapex.com/amd_cpu_mot...lon_xp_oc.html