I've got 145k on mine and the fuel injectors are flaky. It seems to run fine when I've got fuel injector cleaner in the tank, but has a rough idle (in drive) if I don't. I've ordered a new set of injectors et al. and should be replacing them next week.
Lots of threads on rough idles for the M103. I think that's actually a bonus. It seems like there are so many more M103 postings than any other type of engine. Lots of good information out there.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino