With the relatively low torque values of MB heads and the special tool that only allows 90-degrees of rotation at a time, it takes a real idiot to overtorque a head bolt. I suspect someone tried to put a long head bolt in the wrong location.

I doubt valves are the issue if it smooths out beyond idle. A compression test should tell you if the valves aren't seating properly.

How does the economy/vacuum gauge respond to the rough idle?

Sixto
91 300SE
81 300SD