Check a few things...like your oil dipstick...is it foamy and discolored? (best to do right after an oil change when your oil is clean). If not, then check your trans fluid dipstick for discoloration. I just had the same problem with my 1991 300e (177k) and came to the conclusion that my trans cooler in the radiator developed a small leak, therefore leaking trans fluid into the radiator system. I was told by an MB service manager that this is not an uncommon problem. He explained that if your motor oil dipstick is clean then it's the trans cooler. In a bad head gasket situation, I understand that there would most likely be cross contamination...oil in coolant and coolant in oil.

I flushed several times with a radiator flush additive and then replaced my radiator (easy to DIY). I then replaced hoses as needed and refilled with new coolant. A snap!!