I wonder if your EGR valve, or its controls are not behaving right. My theory is that after a long period on the freeway, where it would be open, it sticks and does not close all the way. That creates a vacuum leak and the car stalls. But then the vacuum is zero and the valve closes, no more vacuum leak, and away you go.

Try uhooking and plugging the vacuum line to the valve, and see if that changes anything.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe