Every now and then on a long trip where I am averaging speeds of 100 - 110, the car will suddenly lose power and run rough like it is running on only 3 or 4 cylinders. When this has happened, I have pulled over and turned it off. When I restart, it is fine and stays that way for a spell then does it again. I know that that's not a lot to go on but does anyone have any ideas???

1989 260E