'93 S-15 Jimmy Stopped Runnning.

[Hwy62So]

MY '93 S-15 Jimmy;4.3L Vortec V-6, just quit running one day. When I checked the fuel pump, I found it not working. I replaced the fuel pump, strainer, and fuel filter. It would turn over, but would not start. I replaced the fuel pump relay; still would not start. I replaced the distributor cap and rotor, coil, spark plugs and wires, and all of the rubber vacuum hose's. It still would not start. I replaced the MAP sensor, and it started and idled good. When I tried to drive it, it would only run about 20 MPH before it started missing and backfireing. If you stomped it to the floor it will run wide open until you release the gas pedal, then it goes back to missing and backfireing, until it idles down and runs right. It has no codes stored in the computer. It has been hooked to a diagnostic computer and it showes nothing wrong anywhere. I have had 2 other onboard computers in it and it runs the same way. Anyone with information about what is causing this, and how to fix the problem would be appreciated