1995 chevy cheyenne 4.3 tbi runs great but some

1995 chevy cheyenne 4.3 tbi runs great but some times the check engine light comes on and runs bad have replaced the idle air control valve (would stall when turning fan or ac on ) and just when the light comes on ust replaced the o2 sensor would the throttle position switch make it run rough some times new wires cap rotor what about the map switch? it dosent run bad at all just when the light comes on. the tranny doesnt want to shift smooth ethier it stays at a higher rpm when shifting and hard filter ?