1998 GMC Truck Loss of power while driving

Guys,
To give you a little history on the truck. I recent replaced the 5.7L 350 in my truck w/ a used 5.7. About a month later I replaced the fuel pump due to pump failure. About three to four months later the truck started to loose power while driving down the highway, especially under a load. I've now figured out that after driving about 2-3 miles down the road the truck will run smooth for the most part and then it starts to loose power (slowly have to back off the accelerator). I've learned that if I pull over and shut the truck off for a minute or so, it will start right back up and drive as if nothing were ever wrong. Now I can't put the pedal to the floor at anytime or it will act like it is starving for fuel, chugs or similar to timing being off in an older vehicle. I've replaced the fuel filter, checked the fuel pressure with key on 55-60lbs. Checked running pressure ranges anywhere from 50-55lbs. I have a code for an oxygen sensor that I have neglected to replace for the past year however I have replaced both catalytic convertors when I installed dual exhaust about 2 years ago. I also replaced the coil, another code came up regarding misfire (coil was cheap thought it might work). That is where I'm at and I still have the same problem. Oh plugs were replaced about 4k miles ago as well. Is 50-55lbs of pressure low for this truck, the truck idles fine? It only seems to happen under a load or after the truck warms up 2 or so miles down the road. Any idea.