7800gs+ won't overclock and strange fps

I run a P4 3.0Ghz and Gainward 512mb 7800GS+ AGP (which is a 7900GT chip). However on Far Cry when the action gets tough the frames drop to around 25-30fps maxed out at 1280x1024 4x aa 8x aniso. No probem with that with those specs. So I thought i'd oc my CPU to 3.4Ghz and OC GPU from 450Mhz core to 500Mhz. Absolutely no difference. I ran fraps and got EXACTLY the same result. OK is this a one off? no cos Test Drive Unlimited maxed out at same resolution is the same. I dropped the settings down on this game and the fps was still exactly the same. The only time I got it to change was when running in HDR which it struggled with at high resolution. Oh and i removed VSYNC just in case.

Does anyone have any suggestions on what may be causing a problem? .