The only other thing I can think of is perhaps a small amout of air is getting in.

Also while I have not read it at the below site in the manual there is some sort of modification to the Overflow Valve; in the manual. I have not read it as I do not have that Engine or IP so I do not know the details or what problem it is supposed to solve.

http://www.ps2cho.net/downloads/MB%20CD/W124/Main.html

Random thoughts:
Is it possible that the EGR/Smog system could be causing the Nailing? Too much exhaust gas mixed with the fresh air might be causing late burning of the Fuel?

Is the Engine operating temp normal?

I also remember folks complaining of the inside of their Intake Manifolds getting really cruddy to the point they are difficult to clean.

Also it seems that after 1985 model year or so there is more complaints of nailing. Rebuilt Injectors and Timing set properly usually cure a 617 that has good compression and no air leaks.

__________________
84 300D, 82 Volvo 244Gl Diesel