I used to work at an electric motor rebuilding shop. They did everything from little DC motors you could hold in one hand all the way up to huge stationary motors that you could walk inside and do jumping jacks. The pay sucked and it was filthy. I saw my own blood every day. I spent a lot of time going out on a service truck to steal mills all over North East Ohio. I did manage to get a decent price on about 100lbs of AWG 16 enameled magnet wire for winding coils for passive speaker crossovers though!
We used to spray solvent all over a motor when we were there to clean it. After we washed the whole thing down with solvent we had to blow it out with high pressure air. We had a Meg-Ohm meter to measure leakage between the windings and to ground. Where there should be no circuit there had to be super high Meg-Ohms of resistance. Believe it or not, it would never be infinity ohms. But even many Meg-Ohms will cause a lot of heat when you are talking about the current levels that some of these motors are designed to handle. I have seen very thick steal motor end bells melted like wax!
If I had to guess, there are two things wrong with your motor. One is that it is full of dirt. Another thing that happens to motors over time is that the centrifugal force causes the coils in the rotor to deform outward toward the stator. This can even go so far as to rub, or break the coil wire. Bad bearings is another likely possibility.
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Squat that bug, 
