I have a 89 ford f150 xlt lareint 4x4 with dual tanks and im having fuel problems.

I replaced my rear fuel tank with the pump and then also the the pump on
the rail and then my fuel filter and fuel regulator. I also had my fuel relay
switch replaced and did a voltage test. My truck will get 47 lbs. of pressure
then drop to 7 lbs. There is fuel pressure going from my rail pump up to the
engine. It sometimes will run for an hour then only 5 min. I do have a fuel
switch that seems to be working but in the back where the fuel gets switched
to each tank I am having problems finding that part.I tried to have it
bypassed but no go. I dont use my reserve tank at all since it has a hole .I
have had 5 mechanics look at it and they cant find out why. So hundreds of
dollars later i still cant get it to go. Again every pump is brand new on it and
replaced the new pump in the tank again thinking it could be it but not. Realy
frustrated and just about broke so if any one can help please do. Thanks a
million ..!!!