With latest R326.98, ShaderToyMark runs fine on Win7/Win8 with correct FPS.
Sorry but I don't have anymore a WinXP testbed so I can't tell you if ShaderToyMark still works or not correctly with XP.
Does your GTX 480 work fine with FurMark (correct FPS, GPU temperature)? If yes, forget your issues with ShaderToyMark.
Yes, STM and FurMark work correctly for framerate and temperature despite abnormal performance (STM only) because the framerate by and temperature by Geeks is same number to monitor eVGA Precision.
I can improve performance from 80-90 to 200 fps in STM before bench but "200 fps" isn't normal (due to only max 60% GPU usage and to 26% CPU i5-2500K usage) when the cursor mouse is from stopping to fastly moving, running XP with GF 306.81WHQL driver.
Please, should install XP.
Can you detect any problem found?