This an LPG truck? Might be a loss of pressure from regulator under load...Does this have the Nissan engine with the NIKKI Regulator? If so the Regulator shoud be putting out about 4-5 PSI even unde load,on top of the Regulator is a 17mm hex witha phillips in the middle this is the psi test port put a guage in it and run it into tilt bypass(tilt all the way FWD or Rev) and see if the PSI drops