songs skipping forward with Pioneer CDJ 400

Ive had my CDJ 400's since christmas and haven't had any problems with them until about a month ago. Quite often when I play a track it will keep jumping forward a couple of seconds which makes it near enough impossible to beat match. Normally when I turn them on it doesn't do it for the first couple of tunes and then it gets progressively worse. If anyone could let me know what this problem is and how to resolve it that'd be great.