RPMS fall and I have no Idea why

Ok I have an 87 cherokee and first off the fuel pump whines like a mother after the truck has run for about 15 min. The truck still runs fine untill I hit the gas and the tranny downshifts. RPMS rise then the motor starts stuttering and i loose power. I let off the gas, the tranny upshifts, and as long as I dont put in to much gas it is fine. Sounds like it is starved for fuel, bu here is the kicker. It has a new fuel pump just a few months ago. Any one have any ideas? I dont think its the torque converter or the tranny because the RPMS are falling not rising and the tranny is shifting correctly. If I put it in neutral and hit it, the RPMs rise normally. Thanks for the help.

So, when you put in the new pump did you replace the rubber fuel lines that run from the pump to the tank flange?

What brand of pump did you install?

Sadly I didnt replace the pump. Was Installed before I bought it. It looks brand new. The lines do not look new however. And I have no idea what brand

You might replace those lines, they deteriorate and leak causing the pump to work harder.