I have a 1994 Chevy Cheyenne pick up W/ 4.3 V6,Auto trans, 240700 miles. OBD I system gives code for Idle Air Control valve. Engine light comes on when stopped and then goes off after accelerating. Replaced IAC valve. Engine light comes backs on with same code. Jerks when accelerating at hiway speeds. Changed fuel filter. Replaced Cap,Rotor,Spark plugs,Plug wires,throttle position sensor,and fuel pump. Problem still exists.
If I recall correctly, 1994 is the 1st year for Vortec V6 engines - sequential/port injection - an upgrade from throttle body injection. 1996 is the model year Vortec was applied to V8s. Have the injectors been checked, cleaned or replaced? Could be starving for fuel via obstructed or plugged sprayers. I also wouldn't rule out gas contamination either such as water in the tank.
Time to start checking for vaccum leaks. You may have a hose somewhere that has cracked due to age and is causing the problem.
Scotty has made 2 videos on how to find leaks:
