Hitching (missing) under a load

1993 EB Bronco, 351, auto, push button 4x4, approx. 120,000 miles.
Okay here is the problem, when you first start up the motor it runs fine. When you take it down the road it will run great for about 1-2 miles. Then it will start to hitch real bad, like there is water in the fuel, making it undrivable, then after this starts happening, when you come to a stop it will fluxcuate in idle about 200-250 rpms from 950-1100. So here are the things that we have done to this so far, new timing chain and gears, plugs, wires, rotor, cap, fuel filter, new gas tank and fuel pump. We did disconnect the module to have it timed. Took it to a shop to have them try and diagnose the problem, and they say they have no clue and to let them know if we ever find the problem. Thank you in advance if anyone can help solve this problem.