One thing that got me a while back on this very issue was on my cache OC, I had set min and max to 48.... Everything else was set to let the voltage and freq drop at idle... Once I set the minimum cache to Auto it worked as intended...
Make sure speedstep and speedshift are enabled, and like mentioned, make sure you are on balanced, or that your minimum / maximum cpu control are 0 and 100... I keep a shortcut handy for when I don't want it to drop, I just swap to high performance quickly...
Let us know some of your other settings if you haven't figured it out yet...
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