heres my idea what you need
you need 3 caps & a schoty diode on the motor
schoty diode goes from the pos. to neg posts on motor, sivler band on the diode go s to the + side of the motor if you put in on backwards it just burns up
cap1 put post to post on the power posts on the motor
cap 2 power - post to the motor can
cap3 power + post to the motor can
this should stop all the glitching problems if not you nite need a plastic drive shaft
[&o] soldering to the motor can can be tricky
you may need to sand the spot alittle for the solder will stick to the motor can
oh also if its a new motor i may need to be broke in to seat the brushes
put a drop of oil on motor shaft next to the bushings & spin the motor a few times
this helps alot & makes it last longerfront & back
it may take as many as 7 full battery runs to break it in[&o]
good luck hopes this helps