Always a bit dodgy to diagnose a problem from a distance, but here goes (chuckle);

My guess would be the motor is drawing a lot of current when it is turned on and that is causing the voltage drop. The reason for the excess current draw could be a mechanical bind.

So, to pinpoint my guess, I would suspect the tracks, rollers and regulator mechnism are all dirty and full of 40 year old (hard as a rock) grease. Try taking the stuff aparts, cleaning everything and replacing the rollers and regreasing all the bits - betcha it starts working really well!

Hope that helps!

John P.