[AlexD744]

As a programmer (who has seen motors behave this way many times), I would typically relate this to a software problem. Typically motion like this is described when in one part of the code the motor is being set to a certain speed, while in another part of the code it is being set to a speed of 0.

Admittedly, it could be other things, and as stated above, switching the leads to the motor end of the speed controller could tell you a lot about the root of the problem being in hardware or software.