Timeline times don't match times shown on cursor or preview window

Movie maker 2. New Dual processor PC, 1 Gb ram. Watching an imported video in
the preview window is fine but if I skip forward, the playback picture will
pause at the point I've skipped to, the video is still running as the time
shows until it "catches up" and then the motion on screen continues as
normal. This time delay is not noticable for the first 10-15 minutes into the
movie but after that it it stops for a few seconds and reaches 1/2 to 2
minutes depending on how far into the movie it is. It makes editing almost
impossible. Even with no imported movie the timeline does not match the
cursor time i.e. 10 mins on timeline is 12 mins on cursor, 1 hour shows as
1hr 12 mins on cursor. Is this a clue to my problem?