It sounds like you have cables shorted or open. Other than common source connections there should be no contact between the pump motor and the drive motors.

I will check on the drive motor next thank you for the suggestion. I dont know if this would give any more clues but I have figured out that if I engage the lift so the hydraulic pump is running it will drive forward normally at slow speed, if I release the control so the pump turns off it will stop moving forward...? It always works fine in reverse though...

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.

Check all the contact tips. Really odd that it has this problem in one direction.