Gotta love OP's grammar.
Based on my experience, the most possible reason may due to your video quality is too high (let's say, 720p) that your computer can't render properly. Maybe try to use another one with less quality, 480p is considered acceptable.
The second reason may be because you're mapping with video in the background. Unlike the storyboard which has data stored in files, video is pre-rendered, which makes it hard for the editor to spot where the right timeline is. In this case I'd suggest turn the video off by go to View and uncheck Show Videos.