I actually had this same issue on my 300SDL and found that what was actually causing it was the adjuster mechanism wasn't springing back all of the way after adjusting it to the correct time.

I added a bit of grease to the adjuster and it has been fine ever since.

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1998 E320 Wagon

1992 300D 2.5 TURBO

1991 560SEL M119.960

1986 300SDL TURBO

1992 500SL Parts Car

1991 420SEL Parts Car