It's a desktop...great...have you checked for clog fans and heatsinks on the cpu and gpu? Even though the temps say you're ok but dust/lent buildup on the fans and heatsink can slow the fans down and generate enough heat build up to cause such problems.
Next I would make sure you have all your drivers
updated, in particular your chipset driver
and your gpu driver
Also, could you fill in your system specs in your profile area, this would greatly help with locating the correct drivers and software you might need.