Greetings, I just went through overheating problems myself so I'll put my two cents in, My car is a1987 300 E euro with automatic, I started having problems just like you did and I went through two radiator caps, one H2O reservoir, heater hose, Thermostat, even a radiator which was leaking anyway, and the overheating got better but did not go away for good. Finally what was happening, is that I blew my headgasket, not a lot, but enough to slowly put pressure on the cooling system(exhaust filtering into the cooling lines) like a pressure cooker, so all this exhaust finds it's way to the highest point in the cooling system which is the radiator cap. That is where you hear the boiling sound, the pressure inside the cooling system exceeds the radiator cap rating and you start loosing water until you get vapor lock in the engine.
Do you feel your car a bit harder to start in the morning after the engine is completely cold, because what happened to me was that water would deposit in that piston where the leak was.
So I ended up having to shave off half a mm of the head, machining the camshaft tunnel, replacing guides and seals,reseating valves and so on.... I did take advantage of this to have the engine rebuilt to stantard measurements.
Bottom line is ,It sounds to me like a slightly blown head gasket.
P.S. I didn't find any oil in my water or water in my oil either, the water did smell like exhaust when I opened my radiator cap. MAKE SURE THE ENGINE IS COLD BEFORE YOU DO THIS!!!
Let me know how it turns out.