I have a 96 12 valve with a 5 speed and 215 pump everything is stock besides the fuel plate. when the truck is under load especially in 4th and 5th gear my fuel pressure drops to 10-15 psi and loses power. it has an adjustable over flow valve which is set around 24 psi at idle. while revving in neutral at 2000 rpm the pressure goes up to 30 psi. i have tightened the adjustable ofv before and the pressure at idle would go past 30 but still drop to 10-15 psi when under load. i have changed the fuel filter, cleaned the pre filter screen, deleted the the fuel heater, tried 2 new cummins lift pumps, put in a new fuel line and a new sending unit in the tank but nothing has fixed or helped the low fuel pressure issue. I would like to keep the mechanical lift pump and not go to an electric pump but i cant find whats causing the low fuel pressure issue.