I've run into a similar situation with other types of cars. I found on more than one occaision that it was the water pump in the early stages of failure...
Despite your statement that the water pump is putting out a decent volume, please see many other postings to the list that all say that the MBZ factory water pumps are (relatively) cheap and provide much better flow and volume than anything else.
1989 300 E
1992 190-E 2.3