Assuming you have tried disabling power management in both the BIOS and Windows.
Try installing a specific driver for your monitor, rather than letting Windows use a generic one.
Try installing the latest drivers for your video card.
If that doesn't work, have a look at your CPU heatsink/fan assembly. If it has the pink, hard type of thermal transfer compound on the base, try removing it, then replacing it with the soft white, cream type of compound and refit the heatsink etc.
I have found that this has solved a number of problems, including one like this, with a number of AMD processors, both Athlon and Duron.
Hope this helps.