Pricey

When I have two video clips and having them play side by side, one of them slows down to a crawl after like six seconds of play and the audio disappears as if it is set on slow motion. This makes the rest of the two videos not line up properly as the other plays at normal speed making it very very hard to line them up! I haven't touched any speed settings or anything. I do wait until the buffering bar thing gets full on the preview video screen before hitting play too. Can anyone shed some light on to this please? Many thanks.