Have someone check the camshaft. The early 300SDs did have a problem with cam wear. The updated camshaft requires installing different rockers(with hardened inserts) and valve springs(with a yellow paint marking)to support the added weight of the updated rockers. If you pull the valve cover, look for wear on the tips of the cam lobes. And while you're in there, see if the valve springs have a yellow mark on a couple of the coils of each spring.
ASE Master Tech
Mercedes Tech 19 Years