A "weak" armature can also cause the problems you're describing, wrong crank arm, pivots that are not moving as freely as they could/should.
Normally a three speed motor and two speed switch will give you one speed, usually OH MY GOLLY, and will park where ever you turn the switch off at and not in their desired park position.
On occasion I've seen a brush hanging up in the capture of the brush cap, not being fully against the commutator, give hit and miss run results on a motor as well.
This is the trouble shooting portion of why a motor is not operating properly. If the motor at some point became hot, I've seen the springs that hold the brushes against the commutator "shrink" and not keep the brush/brushes snug against the commutator and that will give hit and miss issues with the motor running properly.
Hope some of this info helps. Have you tried bench testing the motor to see if it operates properly on a bench or not hooked up to the linkage? You might start with the "easy" stuff which is do you have the correct crank arm...I'm leaning in that direction, but I usually start with the easiest stuff and work from there. The wrong crank arm will cause binding faster than anything and will have a tendency to overheat a motor because while you're dinking with it, trying to get it to move again, the motor is still attempting to run and building some heat