Yeah, that was the reason for my overheating problem. I was on the verge of purchasing a new Nissen's radiator when that blue rubber strip coughed up into the expansion tank. I should've updated that thread but it was pretty buried by then.
Feels like a brand new engine now. Power is just as good when it drove off the lot in '91 -- smooth as silk too. Engine temp never goes over 90 now with 45/55 antifreeze/water and a bottle of water wetter. Finally nice to see clean coolant in the resevoir instead of chocolate milk.
'91 300E, 210k miles