Then most likely it gets back to what we were discussing earlier. The motor is wearing out and when it gets hot resistance goes up, slowing it down.
I have a old air compressor that I keep as a back up. It still works but due to its age the tank has corrosion inside, the various air connections on it leak no matter how often I reseal them, and the electric motor builds up a lot of heat. The longer it runs the hotter the motor gets until it shuts down (overload protection). In the past I replaced the bearings inside the motor (they were cheap and easy to do), but that did not help. I even tried cleaning all of the motor's internal components and connections, but no improvement. Its the electrical windings inside that have aged and increased in resistance.
Similar thing happens when starter motors get old.