1982 Porsche 911 Runs rough intermittently

On a long road trip it occasionally began to run very rough at about 3000 RPM in 5th gear on the open road at above 70 MPH, with absolutely no acceleration. I would pull off the road, put it in neutral, run it up to 5000 to 5500 RPM with no problem and could usually continue for many miles running smoothly at high speed with no problem; on other occasions it would run smoothly as long as I kept it below about 70 MPH and 3000 RPM. The fuel pump, fuel filter, coil, distributor cap, and rotor were replaced, CD box was checked and is OK, and for about 1100 miles at high speeds on a long road trip, it ran smoothly, and then began to run rough again. Same symptoms, same temporary cure by pulling off the road and running it up to about 5000 RPM in neutral. Any suggestions on what next? Thanks.