Hi tabletopia,

try reducing the resolution of the playback in the previewer. Open the program, and the quick way to get there is to press the letter Y on your keyboard for program setttings. Click on the Video/audio tab and under Movie display reduce the resolution to half, quarter etc. This usually cures it.

Closing down all other applications whilst you are doing this can also help free the computer, as especially high def material can place a serious demand on your system. Hope it helps. Cheers M o' L