The motor pinion and spur gear mesh is probably too great. There should be about .003 - .005 gap between the teeth of the two gears. An easy way to set the gear mesh is to cut a narrow strip of notebook paper and feed it between the two gears. Then loosen the engine mount screws and slide the engine and pinion gear into the spur gear.
Retighten the engine screws and then remove the strip of paper. You should be able to run a strip of paper between the gears without them binding.
Also remember the gear is made of plastic to strip !, under stress, if the car is jammed ,and revved or held in one position ?
it will strip,this is to save more expensive metal components from twisting and breaking , its like a fuse in the drive train.