how can i fix electric golf trolley?

Recently had problems with controlling the speed of my golf trolley (mocad). replaced switch, gear box and problem the same. finally replaced controller and the problem was resolved for about 6 weeks. now the trolley has a mind of its own. it can accelerate or slow down at any time when in motion without touching the switch. I am guessing its a controller problem perhaps water has got in or faulty controller. I am sure battery, charger, gear box, switch are all ok. I wonder what the problem is as it is getting worse, my golf trolley is out of control on the golf course with these sudden surges of acceleration/ deceleration. its dangerous actually.