Playback flickering between two pieces of footage

I have a clip in my timeline that's flickering between the actual footage and the footage from another video that I imported. The footage plays back fine in the source monitor, and I checked both the original and proxy files for that clip (both of which play back normally). The issue only occurs in the program monitor.

I've never encountered this issue before. Would clearing the media cache possibly help? (and is it a good idea to do so while still working on the project?) What else may be causing this problem?