I have a 1997 Ram 3500, 5.9 12 valve Cummins with 185,000 miles, a few months ago It started losing prime randomly. Once reprimed manually using the plunger on the mechanical pump, the engine started and ran properly. I changed the tank unit, mechanical block mounted pump, and cleaned the fuel heater assembly. Problem still existed. I then installed a 5-7psi electric pump in the supply line before the filter. The problem still exist. It is random. It may start properly five times then the next time I'll have to wait for the electric pump to prime the system. Once started the engine runs properly.
Any information or solutions will be appreciated.

 

I had this issue years ago, it was slightly random but mostly when truck had not been driven in hours. I thought the original mechanical lift pump was failing, but changing it didn't solve the issue. I figured it was leaking fuel from the feed line when the engine is not running. And when the leak is on the suction side, fuel probably won't leak to the outside.
So quick and dirty method was to pressurize the fuel tank, and immediately fuel started pouring to the floor behind the engine. There are two rubber lines (feed and return) between the engine and the frame, and over time they go bad.

Then there's a fuel sending unit. I don't know about '97, but '95 has a metal lines there, and they rust. I had a leak there after I fixed the leak at the front, this time it was the return. Made a slight mess.

 

There's also the o-rings on the back of the motor, but that's supposed to mostly cause loss of prime when parked nose-high. I did this on our '02 a few years ago:
https://mopar1973man.com/cummins/art...lacement-r480/