Adjustment layers become corrupted with "blocks"

I've been having ongoing problems with adjustment layers becoming corrupted and not displaying properly. After working on an image for a while - it never happens straight off - one or more layers will appear as though the mask on the adjustment layer is made up of 'blocks', either hiding or showing the adjustment layer. Please see the attached screenshot to see the problem.

You can see that the 'blocks' also appear on the adjustment mask themselves.

If I zoom in, the blocks on the mask will disappear, along with their effect on the image itself, and the masks will display as they actually should do. But, when I zoom back out, the blocks reappear. 

Sometimes the pattern of the blocks will change when zooming back out; sometimes they stay the same. 

If I purge the history, the blocks remain. 

If I save and close the image whilst the blocks are displayed,  then re-open the image, the actual mask will return to normal and the blocks will have disappeared, but it's a pain to keep doing this, especially when working on large files. 

For a long time, I wondered if it was a problem with the GPU, but I have recently updated the GPU and the problem remains.

It has also remained through PS updates and also OS X updates. Finally, I recently did a clean install of OS X on a brand new hard drive, and installed PS first jus to test for this problem. Hey presto, the problem was still there!! 

Any help will be most welcome as I'm flummoxed!