I have a 1991 300E as well. Was having same issues. Ended up being a head gasket issue. I replaced my head gasket and a few weeks later the engine failed at 200K. Had to replace the engine and now have no issues with the coolant. Do you have any oil in your coolant? It is common in this year and model. There is a small space between the oil passage and coolant passage on the back passenger side of the car between the bottom of the case and head where the gasket breaks down. When I took mine off you could clearly see the gasket failure there. Also, my 3rd cylinder back from the front had a bad coolant leak going into the cylinder. It would have probably held up fine but I was afraid to tighten the head bolts anymore than we had. I should have went back in after running it and retightened them.