Injection pump? Loss of power?

I've had my mighty max diesel for a little while now and it keeps leaving me stranded. Blew it up and replaced the head with a brand new head that came with brand new injectors two months ago. I attempted to take it on a long drive from Denver Colorado to nashville Tennessee after the rebuild and I keep having a major issue. The first leg of the trip I was driving and the truck did excellent until I got to St. Louis and the truck lost power. I mean the pedal just went to the floor with no resistance until I downshifted to 4th then I'd get back up to speed to 5th and then it would keep happening all the way down the gears until I was stuck only going 10mph. I decided to start with the easiest thing I could do, which was replacing the fuel filter. I did so and the truck did great for another 200 miles until it started doing the same thing again but worse and more frequently this time. So I changed the fuel filter again and this time around it seemed to do nothing. I got the the truck to nashville where it sat for a month until I drained all the fuel
Out and I saw that there was a bunch of crud in the tank. I replaced the filter and put Fresh diesel in hoping the filter would help with the issue. I got about 40 miles until that same issue came back up. I ordered a new fuel tank from rock auto and waiting for it to come in the mail.

My question is do any of you think it could be my pump going out? Think that there is air in the lines? Bad fuel?

The truck would never completely die out. It would always idle (very roughly. The needle would bounce around at about 750 rpm. I'm totally stumped!

If it was an air leek it would stop. It sounds like the lift pump is going out. It's the fuel pump before the injection pump.

Is there a lift pump? It looks like the fuel goes from the tank to the filter then filter to the injection pump.

I could have sworn there is a lift pump on these little engines. 95% of the diesels out there have one. I thought they were on the drivers side of the block.