What about sound?

Just a thought, but you do not mention that you are using a sound card, so I figure you are using the onboard sound. Sound processing can use up to %15 of your CPU's resources. I know that after I installed my graphics card, the need to install a sound card became apparent immediately. Not only would the sound skip, as the onboard processing could not keep up with my graphics, but sometimes the video would be affected (as if therer were safeties to prevent video from getting to far ahead of sound in some cases). And the sound issues, often from seemingly benign programs that would never otherwise tax my processer, could cause complete freezes and crashes. In any event, the extra CPU power that you might realize from a dedicated audio board might fix the problem all by itself. If you do not have one already.