Some rewrapped clips are playing back with stuttering or smearing video when users have Perian or avc1Decoder.component installed on their system. Try uninstallign all third party AVC or h264 playback components - you'll find them in /Library/Quicktime or ~/Library/Quicktime.
Some users with newer laptops supporting hardware accelerated h264 playback have problems with black frames or stuttering of some clips in Apple's QuickTime Player 10. Try playing back the same clip in another QuickTime application such as QTPlayer 7. If you're using a MacBook Pro, you can use gfxCardStatus to enable your discrete GPU - this will help narrow down where the bottleneck is.
AVCHD content is, in general, very CPU intensive. If you're using an older Mac, and in particular, if you're working with 1080p60 content, you may not have enough CPU power to smoothly play back these videos. In those instances, we recommend using the "Transcode" functionality of ClipWrap to convert your video to a format like Apple Intermediate Codec. While the resulting files are larger, they require far less CPU power to display.