any time I see an electric forklift throwing "wifs" (weird intermittent failures) I also insist that the customer allow us to get all the corrosion out from under the battery compartment, replace any power cables with cracked or corrosion soaked insulation and add an anti-static/ground strap (especially if this unit does not have a black rubber traction tire anymore) if they want to expect any sort of warranty.
When you say "all the lines ohm out good" are you referring to all the power cables having greater than 100k ohms resistance between the conductor and the frame of the truck when the battery is removed? or are you saying all of the power cables have proper continuity between expected connections?