It sounds like you have come across a known issue with GoPro.
Choppy playback in CS6
http://forums.adobe.com/message/5125072
GoPro http://forums.adobe.com/message/4486784
-and http://gopro.com/software-app/cineform-studio/
Try what is suggested in this help doc:
http://helpx.adobe.com/premiere-pro/kb/playback-stops-loops-soundbooth -premiere.html