I think I've got a new one, and it's cheap to fix (BTW, I've done all of the above fixes on my 500E) but a real pain if/when it fails. There's a coolant return hose going from the expansion tank to the top of the radiator. It attaches to an L-shaped neck that goes into the top of the radiator. These little $2.50 parts can and do fail; the same one's on my 500E and 300TE, and it's already broken on the wagon (with LOTS of coolant being lost, but no harm otherwise 'cuz I caught it) and I'm gonna change out the one on my 500 as a preventative measure.
One other thing: I developed a driveline vibration at around 25 MPH; very annoying, and happened in 1st or 2nd gear. Turns out that the driveshaft center support's been upgraded several times since my car's production (6/92), and replacing it solved the shake issue...car's as smooth as can be.
Good luck all!
All the best, Michael