Assuming that you have tried another cassette, as this problem can be caused by the tape, then the next stage would be to clean the video heads by using an alcohol based cleaning fluid applied with the aid of a small camel haired brush and stroking each head twice in a sideways fashion, (not downwards) great care being exercised whilst doing! If by any chance this action still doesnt cure the problem then the only other possibility is a faulty capstan wheel rubber.