If it's just rough idle for the first 30-60 seconds, most likely you have a glow plug that's bad - although this usually means the glow light doesn't operate as usual (should come on for 3-5 seconds after you turn the key, and turn off when the engine starts). Might be something odd like a dripping injector that leaks a little excess fuel into one cylinder after shutdown, that makes it run rough upon cold start - but that's kind of a stretch. BTW, both your cars have the 3.5L engine that likes to bend rods & cost $6-$12k to repair. If you're not familiar with the problem, do a search on this forum (and the Web) for Mercedes 3.5L problems, oil consumption, oval cylinders, bent rods, etc etc...
