A friend of mine has the exact same fault on his 2000 model, but it is intermittent, with no clue or pattern to the fault. The expert with laptop says its the main control box, but on another visit said it was the Inverter box sat on the motors.
My friend checked all the obvious, handbrake etc, and then asked me to look at the electronics. I stripped the control box and the inverter box down, but have not found any problems visually.
The only way forward that I can see is to swop out the two main items to establish where the problem lies. I did suggest he buys another forklift of the same model, but with a different fault, then I could crack it, but he was not amused!!
Any thoughts suggestion would be appreciated....