Lakeside
While I am modest in my elecrtrical understanding, I do know William of Ockham well.
He requires that the simplest solution that fits the facts is the most likely, and this is the motor getting weary. If you want to take the servo apart and try and scoosh contact cleaner into the motor case. sadly servo motors are always sealed cases so you need to use the fixing screw holes on the front face (assuming it has any)
You could replace the motor - this is perfectly straight forward as long as the new motor has the right gear fitted, but I don't think they are available as spares.
The output transistors (on my dead one) are (or appear to be) FETs, but I would not expect any transistor to "partly" work.
I know this is possible but normally if the smoke is still inside they work perfectly.
andrew