I have been having a similar issue. Although I know why it's doing it, I don't know what's causing it. In my case, it's because the CPU is overheating. The computer will slow the CPU down in order to prevent heat build up. In doing so, the resources are spread much thinner, and less is required to use the CPU to capacity. Does the fan blow hard and does the laptop feel particularly hot? If so, you are likely having the same issue I am. Unfortunately, I still cannot figure out why it overheats.
Edited by hamluis, 05 September 2012 - 02:25 PM.
Split, PM sent new OP - Hamluis.