The O2 sensors get flaky with age - I'd change the sensor (around $100 for the part and maybe an hour labor).

And don't go near a dealership with that car, I suggest (and have done this), that you find a local shop known for their work on Mercedes products for times like this.

Next time, try to get the EXACT P-code is it throwing - Autozone will usually read them for free. We can then go from there.