When your having these problems, there could be something binding in the brush hood which isnt letting the brushes have a good connection against the commutator. Just take your motor out, endbell off and take the washers off each side of the motor shaft. Then you cut the motor, clean that up, then you can spray some motor spray in the motor can towards the bearing, also in the endbell where the brush hoods are. Before you put the motor in, just make sure there isnt anything in the brush hood that could be causing your problem, and get some new brushes in there. Sometimes there can be solder that might have gotten in there. Make sure you put the washers back on both ends of the motor shaft and dont forget to re-oil the bearings. You should break in your motor for about 2 or 3 minutes. This should solve your problem, if not try re-setting your speed-control. Hope this helps.
Team Orion/Team Young Gunz!/BIG Johnson Paints/Long Island Raceway & Hobby