Model and serial number please I'm guessing it's an A1.25XL so will have a sevcon controller in the rear of the truck. Is it monotrolor colum change?? It could be a faulty brake switch, a wiring fault to either the brake swtich, monotrolor pedal/direction control switch or throttle pot, a faulty throttle pot or a blown SCR (I've changed a few), you'd be better pitting the bullet and getting an engineer in to look at it