1999 2500 needing fuel pump?

[write off]

I was wondering if maybe I need to replace the fuel pump. Here is what its doing. It starts out at 50 psi and doesnt matter if you put a load on the motor or if you just idle, it still remains at 50. After 10 min the presure sits at 40 to 45 kinda bouncin a bit. At 15 min you can hold the brakes and put the truck in gear and bring the rpm up to 1400 and then the presure drops to just about 0. At 1500 the truck falls on its face and dies till you let off then it comes back alive. After you let off it takes about 8 or 9 seconds for the presure to come back to 45. I took the pump out already and removed the square screen on the bottom of the fuel pump but that didnt change a thing

ANY IDEAS ARE MORE THAN WELCOME
thanx