You may have checked already, but, confirm first that your battery is watered, clean and fully charged. (negative voltage can occur due to battery corrosion causing the controller to behave erratically).
Next, inspect cabling,wiring contacts and power bars for any obvious damage or opens. Then inspect drive motor condition (be sure to elevate drive wheels from floor to avoid sudden truck movement), check if possible drive motor brushes. Commutator should be dark coppery color with no burn marks or roughness.