If there are variable speed controls, either external or internal, they may need a shot of Deoxit in the green and black can. I've seen controls get noisy and cause exactly the complaint described. Photos I've seen of this machine don't show speed controls but that does not mean that they might be inside.
Its also old enough that if you plan to continue using it, the capacitors in the power supply should be replaced. If it were me I'd replace all the other electrolytic capacitors in the unit too.