Again you need to remeber that you have a 400mhz Ramdac as well as a gpu/shader/mem that are only rated at a certain max speed. The ramdac clock will allow you to go only so high with your gpu/shader/mem clocks. When you start to exceed 1 or more of these clocks then ramdac timings in a way go out of synch. Depending on what clock is driven to high determines what problems you will incur. If the gpu is to high then you encounter artifacts, dropped textures and such. If the shader is to high then you can encounter problems with lighting in sceans or distortions of 3d object renderings. If the memory is to high then you can encounter problems such as tearing, pixlating, color spotting, or lockups.
So as you can see, it's much like a balancing act with the clocks to achieve the best results.
When you said you lost 6000 points at what clocks were you using? Are you using Precision or RivaTuner or MSI to clock with? Are your shaders linked or unlinked with your gpu clock?