1990 4.3 Idles fine. Rev up fine, but will fall on it's face when you you put it under a load. ( drive ). Has new fuel pump. TBI kit. Cleaned injectors. Clean egr. Spark plugs. Cap rotor. Map good. New tps sensor. Timings right. Chased timing wire back to ecm it's OK. Put a can of sea foam in a 1/4 tank of gas ( truck had sat for a while ). Ignition switch? Only place I Can think of that pull the selector would ground out the ignition. Please advise.