You said the radiator cap holds pressure, but have you pressure tested the system with the engine cold to see if it holds pressure and has no leaks?

I had that problem with a 1992 300 E and it turned out it had about 3 or 4 places where it was leaking coolant. The leaks were each very small and hard to detect, at least for me, unitl I pressure tested it cold. One of the leaks was from the o-ring in the thermostat housing and I recall some of the others were just from hose clamps.