Pull both cards out of panel and check them with a magnifying glass. Look for corrosion and solder cracks. This will cause intermittent problems.
Also, what is the battery voltage holding up to under load. Put the unit in lift or tilt bypass and check voltage at panel. Low voltage will do strange things.

i dont know about this truck in particular but

how old is it
is there codes?
what system is it?

have you checked all the coil snubber assemblies?
do any other contactors drop out under load? (pump, line? if so equipped)
is the current limit set too low?


do the f&r contacts drop out when you deadhead the pump while driving?
do they kick back in when the load is removed? or do you have to fully release the pedal or reset the key?

check for voltage drop from neg of the contact coil to battery negative should be minimal when contacts are engaged

clean all the plugs
also clean heavy cable connections with a wire brush

what do you mean by a "load" bulldozing (pushing) loads across the floor can load trucks past the limit
toyotas fry their IGBTs when bulldozing on occasion