I should add that this morning it ran for half an hour with no issues at all.

I checked the connector at the hand control and I get a steady sweep of 5.xK ohms to about 125 ohms in either direction from the throttle pot and the fwd and rev switches both switch on/off properly.

Is there anything in the circuit between the fwd/rev switches in the hand throttle and the contactors that could be dropping the ball?