Try to make your shaders compatible between HLSL and Cg. There is possibility what Cg still compiles better for NVIDIA NV3x chips in comparision to MS HLSL compiler. But there are rumors what NVIDIA will terminate active support for Cg.

For HLSL fx files where are EffectEdit included with MS DirectX SDK. Also IIRC ATi RenderMonkey 0.9 is working with HLSL (but no .fx files).