Hmmm, sounds like you are learning the hard way how not to fix a Mercedes, or any car in fact. Replacing plugs and leads are probably the only thing worth replacing 'because they might solve it'. But really, these 4-cyl engines are remarkably solid and also quite simple compared to newer stuff.
In my 4-cylinder 190E, the microswitch on the throttle mechanism wasn't activating due to incorrect cable adjustment for a few months, and I had none of the problems you described. I did get the exact problems when my ovp fuse was blown. Read this post and i highlighted the fuse:
Car is pinking, which of these is fuel octane adjuster? (pics)
Mine blew from jump-starting the car, have you done that recently?
A *decent* tech (and I have seen so many people on this board with techs who just keep guessing at parts which "might be the problem, mate" and replaceing them at great cost, when if they put any effort into things, should be able to track down the problem to the *1* part that is causing the problem, and replace that one) should be able to help you out if you reach a dead end with your fixing. Like I said, the engine is solid so the problem will be a small sensor or relay most likely.
Sorry, didn't mean to preach, but I get mad when I work so hard to do this on other people's cars when working at my dad's indy garage, and other techs probably make more money selling the customers part after part after part.