More information needed for someone to help... please click below and provide the requested information
-Premiere Pro Video Editing Information FAQ http://forums.adobe.com/message/4200840
-WITH the brand/model graphics adapter (ATI or nVidia or ???) and exact driver version
-WITH have you recently updated to the very latest Mac operating system?
--Mac 10.11 El Capitan has been known to cause problems
-WITH exactly what are the codec details of what you are editing?