Have you investigated the tank at all for the problem ,if never replaced after close to 40 yrs of service this maybe a good place to start.You can start by simple replacing the fuel filter ,cut open the old one after draining and soaking with water ,youll see what appears to be red brownish dirt ,rust .The answer is replacing with new ,replacing with a good reusable one ,or , cleaning then coating your original ,the cheapest route but has to be done correctly.I had to complete this procedure from tail to nose on a 190e , fuel system was nasty ,even succesfully had the fuel distibutor cleaned with a sonic cleaner ,she ran great.New plugs , injectors ,PUMP and filter wouldnt do it ,it had to be the tank with rust resisting the flow and it was in this case.After 4 months of weekend driving Im confident this car is reliable .