Off color question

I have a 7740 Ford/New Holland tractor. Earlier this spring the fuel injector pump started leaking and it would lose power from time to time and only run about half the rpm's it usually does. I took the fuel injector pump off and had it rebuilt. I also replaced the filter head because the primer pump didn't seem to be working, so I got a new one at Napa. That solved my power loss issues, but it hasn't run right since. For starters, it idles erratically. It idles at 700 rpms and will jump to about 8 or 850 for a few seconds and drop back down over and over without touching the throttle. Then if you shut it off for a few minutes and try to restart it, it will start for a second and quit. Try to start it again and it acts like it's out of fuel. You will not start it again until you pump the primer back up. I told a friend about it, and he said the Napa filter head isn't holding prime. I would have to eat it and get another one form New Holland. I would just do this, but here's what's confusing me. When the tractor has a full tank of fuel it runs perfect, no starting problems, on idling problems, until it gets down to about half a tank then the symptoms start again. I thought it was a fluke or my imagination at first, but I filled the tank three times and run it down and it happened every time. If it was a gravity flow system this might make sense. In my mind it either has fuel and it should run or it don't. Maybe it's a vacuum leak, I don't know, any advice appreciated. Ranse

When the lift pump on my gas 4020 stopped pumping my 4020 ran only with the tank at least half full because at that level the fuel got to the carburetor by gravity. Sounds to me like the new lift pump and the filter aren't getting along.

Gerald J.