I have a 1986 Audi 4000S. It does not idle smoothly - at times the idle will suddenly drop almost to a stall but then catch itself and resume a more normal idle. Worse than that, though at times it drives okay, at other times it will malfunction in one of two ways. Sometimes when I come to a stop and attempt to accelerate, stepping on the accelerator does nothing - the engine does not rev at all. Only after I repeatedly stomp on the gas does it begin to respond and allow me to drive. Secondly, sometimes when I am driving the car will begin to "buck" as it often does when a beginner is learning to drive stick, but for no apparent reason because I am going 35 mph and am not touching the clutch.
I have replaced the fuel filter and tried STP fuel injector cleaner, but neither of those cleared up the problem. Does anyone know what is the problem? It seems like they may all be related. Is it fuel pump? Air filter?