Engine Performance problem
2000 Chevy Silverado V8 Four Wheel Drive Automatic 117000 miles

I've had my truck into a mechanic twice now and still have the same problem. I have a pronounced hesitation/backfire from "Off-idle" until it goes into second gear. It doesn't do it everytime, but mostly when engine is cold (sometimes when engine is hot too). I've replaced the fuel pump/sending unit, filter, fuel regulator, plugs, wires, ECT sensor and I have tested all the other sensor's continuity and voltages (except the O2 sensors) and they were within limits. If I rev the engine quickly, I can hear a vacuum/sucking sound that seems to be right near the MAP sensor (using a vinyl hose at my ear). The rubber collar around the MAP sensor is intact, so I don't know what the problem could be.