1992 Caprice idles rough and stumbles when in open loop

I've got a tough one. My 1992 Caprice 9C1 with 5.7L idles rough and stumbles when the system is in open loop. Once the vehicle warms up it runs like a top. The exhaust tends to smell rich at most times. So far I have replace the Idle Air Control, Throttle Position Sensor, EGR Valve, MAP Sensor, Oxygen Sensor, all of the vacuum lines, Spark Plugs and Wires, Distributor Cap and Rotor, Coil, Module, Fuel Pump, and Fuel Filter. All have had little or no effect on the cars condition. The car has had this problem for some time and continues to occur even after my father and I put a used but newer motor in it last fall. The motor that was put in was the same size and type as the one removed. The car never displays a trouble code, and again runs fine once the car warms up and switches to closed loop. An interesting note, I happened to have a computer from an older model truck with the same motor, so I swapped that out with the car computer and it runs fine at all times. Thinking the computer was the problem I purchased another caprice computer and it went right back to doing the same thing. While the truck computer runs better, I would prefer to find the problem and be able to use the police computer that either came with the car or the additional one I purchased as it has better performance and fuel mileage. Thanks in advance for any suggestions anyone may have.