At least in my 87 sdl, there was a fair amount of literature that described surging with the cruise control at highway speeds. I never noticed it, but supposedly it is a problem (maybe like you describe).

My vacuum lines near the switchover valve that go to the charge air pipe and the vacuum amplifier were completely shunted or incorrectly routed. I read some about simplifying that vacuum wiring (can't remember where) and modified the lines to closer to the original.

One of the diagrams that I followed was from the CD.

Maybe that helps....

alec

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84 300TD - 235k - Farbe "Surfblau" bought at 213k
87 300SDL - 131k - Farbe "PimpGold" bought at 115k
00 Klepper Faltboot Expedition Double