I have not looked at a 750, but many of the Kenwoods had a speed stability problem created by aging/oxidized/corroded switch components under the 33rpm/45rpm speed control buttons, which on many units was a toggle type switch.
If you play with putting subtle changes in pressure up and down on the speed change button(s), and observe crazy speed fluctuations, then a thorough cleaning of the switch internals may solve the problem.
If you play with putting subtle changes in pressure up and down on the speed change button(s), and observe crazy speed fluctuations, then a thorough cleaning of the switch internals may solve the problem.