i'm not assuming anything as there are myriads of reasons that the problem can occur, transcoding issues, network issues if streaming over wifi, and just the app doing a bad job with certain codecs (mp4 is just a container, not a codec) --- what i do know is that some of the files i play with beamer accumulate a bit of a buffer lead, like when you pause the movie on the apple tv and some of the progress bar is white, indicating there's a bit of room where it's downloaded in advance kind of like youtube does...... those tend to be h264 movies... whereas some xvid avi files i've used beamer with show nothing in advance... so if wifi was to say drop below an acceptable level for a fraction of a second, i think that would be enough to trigger a buffer cogwheel