I've seen the ballast resistor do crazy things on the older C500 Clark forklifts. I think that is probably a good next step.

If the resistor doesn't fix the problem check your grounds, I've seen bad grounds on the old Clarks mess with you too.

Easy way to check your ground is to just put a jumper cable (black end) from negative battery post to your engine. If your problem goes away then you have a bad ground.