1989 Chevy Cheyenne Code says oxygen sesnor rich

Here is the problem: Truck runs high idle at start up, and sometimes it will go down as it warms up. When in idling in park before it is warm there is occasional misses (spit and spatter). When it warms up still in idle it misses even worse and will surge from high to low idle and at times has stalled. When driving there is a noticable loss of power because it constantly wants to stall. The missing and sputtering happens at all speeds. There is a distinct rich exhaust smell. Here is what we have done so far: First we scanned the computer and it read it was the O2 sensor running rich. We replaced it and gave it a tune up. In the quest to eliminate further possiblities we also replaced the MAP sensor, cap and rotor, ignition control module, electronic spark control, TPS, EGR valve and gasket, IAC, engine coolant temp sensor. In addition we have checked the timing with a timing light. Also we have had 3 different throttle bodies and the problem has never changed. We also put another distributer in, no change. We changed the fuel filters (both internal and external) last year due to another problem. Fuel pump was also replaced. After all this and scan still says running rich! After the open loop closes there is a distinct difference in the operating. What else is there? Please help!