Richard, whats this about the keys? i have a 1973 280ce with the same sounding problem, haven't changed any sensors, just finished replacing the pump,filter and fuel lines and hasn't changed anything. I only have one key for it or would this be a far later idea then a 1973 model?

RonD, just wondering if your problem is the same, have you tried to clamp the return fuel line? if i clamp the return line my runs a treat, just can't work it out!