I have a 93 300D, had the same problem, car would start well then after the glow plug relay clicked off the car sounded and felt like it was running on two or three cyl. for a minute or so then ran perfectly, White smoke, definitly unburned diesel, no coolant leaks or head gasket problems. My mechanic said this was quite common on higher millage 2.5 turbos, problem is you have one or more slow leaking injectors, overnight the faulty injector bleeds down fuel into the cyl., during pre glow there is enough heat to burn most of the fuel, heat off overfueled cyl. no burn rough idle until the fuel is cleared, I would drive 100 ft and the car would then run perfect. I removed all injectors and had them rebuilt by a Bosch certified shop, had them all opening within 5 bar, 10 bar is min. spec, no more leaking car idles when cold as before like a sewing machine very smooth. Good luck hope this helps
Ron
2003 E55 AMG
1999 E320 WAGON AWD
1998 E 300 D SOLD
1993 E300D 2.5 Turbo
1985 300 TDT
2004 E320 CDI on order