This had been a problem in the last couple of versions of Photoshop. I previously thought it was due to an old system and video card. However, I have updated my computer and installed a new version of Photoshop and it still happens at the same rate (about 2-3 times a week). Without any consistent reason, the video stops updating. Note, if I keep working blind, the changes, that I can't see, are applied to the image. All other software continues to work correctly. Often, closing and opening the file fixes the problem. On occasion, I need to restart Photoshop.