Just read your post, and I had this same exact problem on my car. My problems were when I tried to start it after it sat for awhile, it was shake, shimmy and get all crazy for about 5-7 seconds, and then smooth out and everything would be A-OK.
I took it to the dealorship, and the first thing that the tech. though of were VALVE SEALS. I have the M103 (190e 2.6), and they're known for getting old and brittle.
He replaced the seals (about $275), and it's been like a gem ever since -- no hard starts any more.
I seem to ALWAYS suggest valve seals when I post ... I'm bound to be right sooner or later!!