It isn't recommended to play MOV files directly from XPression. They should be converted to AVI's using the XPression video coder. Certain codec's of MOV's will play OK (such as MJPEG encoded MOV's) but most formats are not playable in real-time. Which codec is your MOV file encoded with? The open source tool MediaInfo (find it on google) is very good at giving you the details of your media file.
Also, you need to keep an eye on the performance meter at the top of XPression. If you are trying to play too many HD video clips at one time the performance meter can get very high. When it is over 100% you will start to see stuttering. Its generally recommended to play no more than 2 fullscreen HD clips simultaneously. Some very old machines could only play 1 at a time.
#XPression