Sounds like the rheostat. My 49 did the same thing. Like Ritchie54 I gave mine a shower in contact cleaner (similar to the linked electronics spray) then gave it a rigourous workout off/on, off/on etc. This fixed it right up.
This is a common problem w/many old rheostat controlled devices, especially old radio/TV volume and tuner controls.
A quick shower and workout generally makes them happy again.
Contact cleaner is best as it leaves no residue. Residue can short out all that hard work
Good luck
Dave
