Originally Posted by G33Kster

If it is a brushed motor, the problem could be a broken brush spring. I had this same issue on one of my cars before I went brushless. The spring that pushes the brush onto the comm probably broke and the brush is no longer making consistent contact to the commutator. Chances are, the broken piece of the spring may be stuck to the side of the motor can
In any case, you'll have to go to your hobby shop and buy new spring(s), they're cheap and relatively standard across all cars and motors.