When this used to happen on my old tape players years ago I would attend to the heads and tape path - dirt builds up over time and causes erratic tape movement - video is always the first to go - sound is more forgiving.
Put it on the kitchen table - take the lid off and give the tape path a gentle clean with an old toothbrush and some alcohol - not the sort you drink - the sort you clean things with.
Good luck...
Peccavi, February 2013
