Here is the latest.
I pulled the rad, and it looks very good - no corrosion, gunk, etc. But when I backflushed it, I got three plastic parts out of the top left inlet. They are all nearly identical, and look to be broken off of a common part which has a dia of about 13-25 mm. these pieces would allign to centers.
I flushed the rad, and reinstalled. I think the car is running a bit cooler, but it's too soon for me to have a firm observation. I also think that now my windshield washer heater has low flow. I thing that based on the CD coolant circuit that it should get flow all of the time.
Say tuned as I work this mystery.
1995 E320 Sold at 288kmiles