First of all make sure your batteries are charged well enough to run the truck, if they are discharged it can cause current overload and make the system trip out due to high amp draw.
with that said...
now...
from the sounds of the symptom it might be a PMT fault.
check your directional contactors, if they are worn out or burnt up chances are they are not making a good connection or if the armature bushings in the fwd/rev contactor are worn out it is not tracking straight and the contacts may not be making correctly. This would be my first guess....
second guess might be a faulty scr on the control panel.
Usually it might be caused by a 1REC being shorted or the 1A contacts being welded together or in some rare cases the 3REC diode being shorted.