I've known bad Hugo style couplings start to lock up, quality can vary a lot. I agree with Malcolm about the 1 mm being rather a lot of free play, just enough to allow free rotation, no more.
I would check the motor directly on a battery, ( assuming a brushed motor) if possible put a light load on it, a pulley or similar would do. Check for any sparking. If you have an ammeter, see if the current changes significantly over a few minutes running.
All this is assuming that the prop shaft, lock nuts, alignment etc are fine.