1983 300SD intermittent rough idle

[83w126]

I have tried adjusting the valves several times and changing the fuel filters, but it doesn't make much difference. But, sometimes (especially after getting off the freeway, but other times too) it will idle so rough the the a/c lines are hammering against the frame, and the hood star (along with the rest of the front of the car) shakes side to side an inch or two, then at the next stoplight it will idle almost as smooth as it probably did when it was new. Is there any way to fix this?