I had the same problem in my 190E and took me six months before I found a non-MB dealer mechanic to fix it. He told me that it could be either "idle speed valve" is bad or something else. That something else turned out to be the 1/2 inch upper idle speed valve hose that was cracked and leaking vacuum. This caused my engine to go from 500-1500 rpm back and forth and stalled. When in traffic I had to keep my foot on the accelerator and one foot on the brake to keep the engine from stalling.