Air/Fuel system problem-runs choppy at low rpm and stalls at red lights

[MazterDizazter]

I own a 1988 V6 Camaro, with a 6 month old Flowmaster cat, new plugs, wires, distributor cap and rotor, throttle positioning sensor, fuel pump, fuel filter, etc. When I start the car and drive it, pressure builds up in the gas tank when the gas cap is on tight and raw fuel goes through the line into the evaporative charcoal canister, causing it to run choppy at low rpm's and stall at red lights. My father and I have changed the canister, fuel pump and filter, and replaced the TPS. There is no problem with the TPS, O2, or MAP sensors, and the problem still exists even without the gas cap on tight. We suspect that it must have something to do with the fuel pump, can someone with experience PLEASE help me so I don't have to sell this thing for chump change? Thanks