Most likely a mismatch between video and editing sequence
The NEW ITEM process will ensure that your editing sequence matches your video file
CS5-thru-CC PPro/Encore tutorial list http://forums.adobe.com/thread/1448923 has a link to learn about the NEW ITEM process, including a picture to show you what to do
Also, more information about your computer might help - Premiere Pro Video Editing Information FAQ http://forums.adobe.com/message/4200840
Try dropping it in a 25/30p timeline.