1997 Ford F150 Rough Idling

I have a 1997 Ford F150 that threw a #7 & #8 misfire. That was 2 1/2 years ago. Check engine light has been on since.It has been in the shop for over 2 months in that period of time and problem not fixed. It runs great at any speed until you get to a red light or stop sign then it starts running rough off and on. If you have any load over approximately 600# it runs worse at the stop light and signs. At some points it acts like it will shut off but put it in neutral and it stops. Once you put it back in drive it goes real slow then speeds up through the intersection. It has shut off a couple of times. When towing it drives extremely well but does the same thing at stops.Here is what all has been checked or replaced.The spark plugs have been replaced 2x, both coil packs 2x, engine fuel system been throroughly cleaned 2x, idle air control valve replaced, fuel pump relay, air filter and oil changed regularly, torque convertor and transmission has been throughly checked and ok. Do you have any ideas at all. Ford has told me that it could be burned valves at one dealer and the other has said I have (warped) cylinders. I am tired of paying money and never get the correct answer. The truck is in overall perfect condition otherwise. Please give me some advice. Thanks!!