I have been using Premiere Pro CS5.5 and I've noticed that when I'm working with the program it will work fine up until the point that I need to launch another app. For instance if I need to do something to a clip with After Effects, Premiere will play the timeline fine until I actually switch to AE. When I try to go back to Premiere, and I click play, the preview just won't play. This happens whenever I switch from Premiere to any other app, even the Finder. The only way to fix it is to restart Premiere, but as soon as I need to switch to another app, the problem comes back.