You have already done the nVidia control panel, Manage 3d settings, power management mode, prefer maximum performance, so I will assume we're talking Windows 7, and will assume you have the gpu priority set to low and the smp priority set to idle, so that a priority conflict isn't the cause.
You probably need to try using riva tuner to force 3D clocks:
Click on the Power User tab, then click on the plus (+) sign next to Riva Tuner\Nvidia\Overclocking.
This expands the menu, double click on the box next to EnablePerfLevelForcing and enter 1.
Now go back to the overclocking menu, the Force Constant Performance Level box should now longer be greyed out. Check the box.
A reboot will be required once you select performance 3D and if so desired, you can always disable it when not needed.
In nVidia Control Panel, you could also try adding a Program Setting for the Fahcore_XX processes and select prefer maximum performance there instead of using the global setting that doesn't appear to work.
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