Maybe, guessing here, you have a problem with one of the servos that rotate/align/stabilize the disc.
If, the speed is not constant/linear etc, it will affect the playback of a film.
Without really seeing or hearing it, it's difficult to form an opinion.
You may have to bite the bullet and get it looked at.
It's not so drastic as you may think.
£30 or so will tell you what's wrong at a workshop. Then you can decide what to do. It's up to you if it's worth it.
Good luck.
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Douch!!......Wot's da madder, da CIA got you pushing too many penzils?
Work:
www.hughes-tv.co.uk