I don't know if it's a continuously variable speed, but if so the potentiometers get corroded with age, as well as dust. A squirt of a suitable contact cleaner may fix it.
Electrolytic capacitors age - if there are any in the speed controller, it could be the problem.
I am planning to do some scroll sawing with the goal of making a wooden gear clock and automatons. Before that I will try some simpler things - shapes, puzzles and maybe some intarsia or fretwork. I have no idea if I will enjoy it or be any good at it!
A friend has offered to lend me a scroll...