Anders M,

This is due to the electronic system fail safe. System being activated, when the direction contact pulls in to drive the truck If the system detects a fault it blows the contact out and the truck appears to leap, there may be a short on the wiring or the logic unit may have failed a board component may be failing possibly one of the three thyristors has failed or one of the flywheel or braking diodes has.
The system on this truck is known as the mark v second system,
If you take the lid of the logic unit you will see that the unit comprises of a base board with four other boards plugged into it also several relay assy all these can be removed carefully to be examined for burnt out circuits,
Check the shorting contractor make sure it's not stuck in as this would also cause this problem.

Regards and good luck Titus