Recently there are some strange issues with my 1987 300SDL
This morning upon starting the engine, the tachometer did not work.
After the engine became fully warm in about 10 minutes, there was a sudden noticeable surge in the engine speed and the tachometer began working again. Then a moment later the same surge happened and the tach stopped working. So this goes on back and forth. The two seem connected obviously.
I don't think there is anything wrong with the tach. What could be causing the sudden engine surge has me most concerned and how is it related to the functionality of the tach. Any knowledge on this matter would greatly be appreciated.

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1987 300SDL