You really must have some type of supply problem as a non dampened pressure gauge slams against the stops if things are normal fuel pressure wise. Your indication from 0-5 pounds was very poor. That slaming occurs when reading the pressure after the second fuel filter. You cannot read the gauge at all the needle is moving so fast. When the injection pump elements are being supplied properly there are strong reflected pulses present in the injection pump causing that.

The water pressure gauge is usually always a non fluid dampened unit. If you cannot see an air bubble floating at the top of the liquid internally it is not fluid dampened. I think you really need a proper gauge to try to establish what is right and what is not.

The lift pump by design should not be intermittent. Yet once again anything is possible. At this point with so little valid information it might still be very weak though.

A good lift pump will give 30 lbs pressure directly into a gauge. You cannot hurt the pump by stopping the flow for a pressure check with a gauge. Your crude gauge will accuratly read the lift pump output pressure I think. Once again a dampened gauge would be better.

You are running out of things to check and might have passed over the problem already. Even worse is you do not yet have any valid indication of how much pressure or quantity of fuel is there.

Try switching the return and supply lines from the tank up front if you have more than 1/4 tank of fuel. This will clear from the front to back at least. Do not neglect to prime until fuel comes up the return line. Things have to be positivly eliminated in a fashion that leaves no doubt at this point. Otherwise it may not be solved. Or it will become very painful to do it.

Something is desperatly wrong here. I thought when you switched injector pumps you switched the lift pump at the same time. It is bolted to the pump and normally goes along for the ride.

The wild card is still the use of different fuels. This means a lot of junk probably got loose in the system at some point. Thats the main reason I suggest switching the supply and return lines. Even fuel lines become obstructed with the resultant crud. Valve seats in the lift pump are not immune either.

When I suggested you feed one type of fuel to the injection pump and then the other type for comparison originally. I meant feed the injection pump. Not the lift pump as a lift pump could get a little more pressure going with vegatable oil as it would seal the valves in the lift pump better than the much thinner diesel fuel. Your aproach may have led to an unrequired change of the injection pump. Or I was not clear enough.

Do change the lift pump though before proceeding as you mantioned you where going to. It is still a good candidate from what I can tell so far.