thanks to rcav8tor for sending me a main controller card.cured everything that ailed this truck.
all is working fine and he even sent me spare voltage regulator,pump control and f/r control cards to boot to ensure this nearly 30years old machine will keep going!
thanks a million.
battery capacity was never an issue in this case as contactors would drop out the instant accelerator was touched without the motors even having time to spool up.also,the different behavior of the truck depending on the position of power/economy switch and steering switches indicated greater issues than supply power.
nissan's service manual also dictates that drive tests be done with truck lifted of the ground.
will be sending my old card for rebuilding now and keep as a back up.