16 valve stalling

My 1986 190e 2.3-16 has a stalling problem. It occurs at operating temp and only when the weather is 90 and above, even humid days. As I drive along and press the clutch to disengage the trannny at a stop light etc, the rpms drop to less than 300 then go up to normal 1000 rpm. Sometimes the car stalls completely. It starts right up though. This gets worse with the A/C on. Any assistance would be appreciated. Could it be a fuel pressure problem? The car has the original fuel pump and accumulator. Only the fuel filter has been replaced at regular intervals. I also think that it might be a vacum problem. Could it be a fuel pressure problem? Your assistance would be appreciated. Joe