Help! Premiere won't allow playback of my video track

I'm having an issue with video playback. When I make a sequence and add any sort of clip or audio to the track I can scrub through the timeline, but when I initiate playback of my sequence (spacebar or the play button) the playback window freezes but I can still scrub through the sequence . The vectorscopes and audio levels freeze as well (probably because I can't play anything back). Does anyone know a fix to my problem?