The blower motor in our forge ( controlled by rheostat) surges at mid range setting. Does this sound like a rheostat problem or a motor problem? 

My guess would be the rheostat.  Take it apart, do a visual inspection of the winding and clean it.  Or just replace it.

If you would edit your profile to show your general location, you may be surprised that there might be members near enough to visit and lend a hand in fixing the problem. It might help to know the motor and rheostat make. 

I can’t control the wind. All I can do is adjust my sails. ~Semper Paratus~

motors and rheostats are not good mix, try a fan speed controller

Am I the only one who calls them "potentiometers"?

Two terminals vs three.  Thanks, Grok!