PROBLEM 1: BLACK BOXES ETC
Black boxes that appear and disappear randomly on your images, something like this:
Not only black boxes, though. Sometimes white boxes, or weird blue colours in the Liquify screen, or image disappearing when you draw a crop marquee, etc. Annoying stuff. Never problems with the image itself, just with the way it's displayed.
SOLUTION A: Go to Edit>Preferences>Performance, and click on the "Advanced" button. Change the Drawing Mode to Basic, then restart Photoshop. In fact, restart your whole computer if you haven't done so recently.
SOLUTION B: If Solution A doesn't work, go back to Edit>Preferences>Performance, and uncheck the "Graphics Processor" checkbox to turn it off altogether, then restart Photoshop again.
PROBLEM 2: TERRIBLE GRAIN
When your photos look awfully grainy or noisy, but when you zoom in to 100% it goes away.
SOLUTION: Go to Edit>Preferences>Performance, and check your Cache Levels. You'll find they're on 1 or maybe 2. Change them to 4, then restart Photoshop (or your whole computer if you haven't done so for a while):
DESPERATION:
If none of the above solutions work for your issue, fill this out then post the results in Ask Damien, along with a description of your issue, or better still a screenshot to demonstrate it.