Won't pull away from rest, but then runs if kicked

[The Pookster]

During a wet run yesterday my B4 with LRP X12 and Sphere TC stopped on track. Now it won't pull away from rest but if you give it a kick it runs OK until next time it comes to rest, then same thing happens.

Swapped the sensor lead and stripped and cleaned the motor but still the same. The speedo LEDs come on as expected but the motor doesn't turn.

Speedo seems dry, haven't got a different speedo or motor to try.

Any ideas?