I've recently upgraded to a new computer (running Windows 7 x64) and CS5 (from CS4). I've got an NEC Spectraview Reference 2690 and I'm having trouble with linearity in Photoshop only. My working space there is ProPhoto RGB, and when I create a test gradient in that space I get colour banding and poor linearity. However, if I convert the gradient to AdobeRGB (or sRGB) or grayscale (2.2) I get nice, buttery-smooth linearity. The gradient also looks great outside PS, as my desktop image; ie viewed through Windows's native WCS system.
I calibrate to Gamma 2.2 using Spectraview (BasICColor) and an i1D2 and always get very high-quality validations. My best guess is that PS is somehow applying the ProPhoto profile twice. Is this possible? Can it be fixed? I realise this may require some knowledge of Windows's arcane systems to resolve, but any suggestions are very welcome.