Images grainy or pixelated until I zoom in

Hi

There could be a number of issues - including your GPU, and a screenshot would help identify but:

If the image is 16 bit and changes dramatically between 66.7% zoom and wider then you may just be seeing the impact of Photoshop using 8 bit previews to blend and render zoomed out images, even when the overall document is in 16 bit. The effect on most images is minor but, on some, it can display a very visible change. The correct view is the 100% zoom view which is blended at full bit depth and has no interpolation applied.

If it is not that, please post the screenshot requested by c.pfaffenbichler. To do so use the blue reply button next to your original post, and then use the image attach button.
Dave

Could you please post screenshots with the pertinent Panels (Layers, Channels, Options Bar, …) visible to illustrate the issue? (Including one at View > 100%) 

Does turning off »Use Graphics Processor« in the Performance Preferences (Photoshop > Preferences > Performance > Graphic Processor Settings) and restarting Photoshop have any bearing on the issue?