I have never had a Star cart, so large grain of salt here.
I empathize as intermittent issues are so difficult get to root cause.
The fact that is slows, but still moves is interesting. For kicks I would first check all wire connections to be sure they are clean and tight, especially the large wires on the battery pack, controller and motor.
If all wires look good, I am in over my head without a wiring diagram. The game becomes measuring voltage at various places, motor, controller (input/output side), potentiometer (not even sure Star has one)
, all the time waiting for it to fail.
If you find a common fact that is true every time it fails the issue may get easier to isolate.
Good luck, I wish I could be of more help.