Just a thought over here, it may not help, but it might get some explanations.
Just after I bought my new cat converter last November I was getting sulfhur oxide smell. I figured that it was the converter breaking itself in, but it persisted up until I had the rest of my exhaust system changed, now it doesn't stink anymore.
I figured that it was the O2 sensor not doing its job, or the plugs or fuel filter or something not burning the fuel properly.
Not to take away your steam Allmachtage, I'm just throwing kindling on the fire and seeing if anyone else is having these problems that we are.
Diamond White Classic Aurora - Back on the Road
Galaxy Silver Grand Prix GTP CompG - Soon to be mothballed