This is gonna sound weird, but stick your finger up the exhaust pipe, and see what ends up on your finger. If its black/sooty, you are definately having a mixture problem. So I suppose your 'mechanic' is on the right track as far as fuel related issues goes (unless your timing is off somewhere in Timbuktu) I'm kinda wondering that some sort of "check engine" light hasen't reared its ugly head. Alot of people like to blame O2 sensors for rich mixtures (including me...I have had to replace several of them in my past cars), and if I had to take a stab at diagnosis just for the heck of it,, I would have that that sensor tested.
I just realised your in Georgia...take it to a Benz mechanic with experience, you have plenty of them up there. (like the fellas that have record amounts of post for instance