8600 GTS overclock problems

I'm trying to overclock my video card (XFX 8600 GTS PCIe) using the nTune utility. I cannot overclock it even by one MHz. i can't even underclock it!! Every time I try it says the overclock failed and resets itself back to factory defaults. Anyone have any idea why it won't work?

nTune is a poor tool really, the best two are RivaTuner or ATiTool, the latter has an artifact scanning tool.