On my 1998 E300, I had lumpy idle, but only on a warmed up engine, and only with no load. Even just turning on the A/C compressor was enough load to make the engine instantly smooth. Turned out to be worn out IP. I replaced the IP and that solved it.

__________________
Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2015 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel