1990 Volkswagen Cabriolet Rough idle and stumble on acceleration

1990 Volkswagen Cabriolet mileage: 74,000. Rough idle and stumble on acceleration. Engine idles best when warming (not perfect), but becomes worse at full warm up. At normal operating temperature, there is an associated popping perhaps pinging sound with the poor idle; also, one tends to think engine could be missing at idle. Engine appears to run fine at or above idle (above approx. 1200 to 1500 RPM). Once going, the car runs nicely at highway speeds. Things that have been replaced trying to correct the problem are: Distributor (intermittent miss was detected from old distributor on engine analyzer), cap, rotor, wires, plugs, fuel filter, fuel pressure regulator, oxygen sensor, water coolant temperature sensor, and air filter. Timing, and idle adjustments (idle bypass adjustments) have been made a number of times. I would like to add that I think I hear an exhaust leak (exhaust manifold). The noise is most audible when starting the engine when it is cold. Also, I would like to add that I have run injector cleaner in three tanks of gas. Even sprayed considerable cleaner in the throttle body to cleanse the intake area (at times I thought this showed idle improvement). Any ideas would be appreciated. At this point, I am wondering if a slight exhaust manifold leak is causing oxygen sensor to send error signal to computer. I have gone full circle a couple times so I would appreciate your thoughts.