Disable Nvidia driver 2D underclocking?

I run FFT3DGPU in my Avisynth, which taxes my 7900GT heavily. Under normal circumstances the Nvidia driver downclocks my 7900GT from 500Mhz to 300Mhz, which cuts my encoding speed by 25% (filter chain choked at GPU).

I remember some version of the nvidia driver allowed the user to set separate 2D/3D clock. That have been disappeared from the newest driver :S I tried changing it through RivaTuner, the max I could set is 468Mhz in 2D. It gives a small improvement but still not enough.

The card goes up to 500Mhz when I started my encoding, and after a while it drop from 3D to 2D clock (stated in Ntune monitor).

I also tried overclocking (i.e. 505Mhz) in Ntune but the driver still go down 2D speed as it think the card is idle (while it is actually working at 100% load)

If I turn on some 3D application it might bump up the clock, but almost all the graphics memory (256MB) have been used by the avisynth filter :S I even had to reduce the screen resolution to 1024x768 just to get the filter running :angst:.

Running the Non-GPU version of the filter will slow my encoding speed by half :juggle:
Could anyone kindly point out how to disable downclocking at 2D?