While running anything that stresses the CPU, SpeedStep is intelligent enough to keep the processor running at maximum frequency. Your FPS issues aren't due to the processor.
To completely stop the processor from going into low-power idle mode, you need to disable SpeedStep, C1E, and all CStates settings.
Check out the Official ASUS P8P67 Series Overclocking Guide and Information
for more info.
Edit: My Sandy Bridge system also spikes from idle mode to full speed every few seconds when I'm not actively using it. This is normal behavior, as different things are going on in the background that we are unaware of.