how is the floor where this unit works? dirty? dusty? metal shavings? potholes that may 'stall' the truck if a wheel is caught in the hole?
Does this unit have 2 each Curtis 1244 controllers? these controllers are very sensitive to bad motor brushes or 'toasted' armature-commutator bars, where arcing has caused a rough spot, or a stalled motor has caused raised commutator bars and causes a 'brush bounce', and the way these motors are mounted causes/allows a lot of dust and debris to get into the motors especially if the vented brush covers have been left off, but still true if they are left on.
Also, on these controllers, the molex pin & sockets have been known to cause problems with intermittent connections. if it was me, I would tell the customer to expect that both motors need to be rebuilt by someone that rebuilds forklift drive motors (not just your regular electric motor re-builder, there are some things about a forklift drive motor that are different than regular A/C fan or IC engine starter motors, like the fact they turn in both directions) and they should consider getting the kits to replace all the pin-sockets and housing of the molex connector, [or eat an entire wire harness expense, which {my guess} is more $ than the resale value of this unit].
and of course you are going to fix every power cable that has gotten contaminated with battery corrosion, or gotten the insulation cut by rubbing.