It sounds like the sensor is only making intermittent contact. One of the first things is to check the alignment of the probes which are fitted behind the crank (they are mounted on a plate fitted to the bottom bracket). They should be gently touching the plastic surround behind the crank. Adjust with gusto if not. Try again, and see if problem is solved.Afternoon . Bit new to all this so bear with me. Just bought a secondhand um 36 urban mover, and the motor only seems to cut in every couple of turns of the crank. Any advice on what I need to replace/repair please? Thanks