Prolink 6600 GT artifacts in 2D - cannot test at 500MHz clock (default)!!

[imported_jerryw]

Hello all. One week ago I bought a Prolink Pixelview 6600 GT. The default core clock for both 2D and 3D is 500 mhz, but in 2D, 500 MhZ create artifacts and small freezes. Lowering the 2d core clock to 465 works just fine, but testing for more than 465 in Forceware gives me an error that it cannot test to this frequency, eg. 470,480,499 , and it asks me to lower it (!!!). 3d settings though work perfect and pass the test. What could be wrong with my card? Any ideas would be appreciated. Thank you