I have a '78 300SD and had my radiator recored this past summer. While I was in there I changed all the belts, hoses, tranny cooler hoses and oil cooler hoses (they showed signs of wear). I also changed my fan and fan clutch for optimum cooling. Did everything just shy of the water pump. I also changed my alternator as it was showing signs of wear and seemed that it might fail soon. (It was original). I also installed the nine blade cooling fan instead of the 6 blade.
When I was all done, the car was perfect. Summer stop and go traffic - 100 degrees outside, the car would run 180 - 200 degrees which was great. Then three weeks later she was as hot as hell. Couldn't understand it. Checked everything multiple times! Even pulled the radiator and had the recore checked.
Guess what, I didn't mount the fan shrowd properly and it hit the nine blade "plastic" fan and shaved some of the tips off slightly. Put a different fan blade in and it was perfect again. Taught me a lesson!
Also, listened to some advice to leave my water pump alone - didn't you know it if the pump didn't start to fail about 500 miles after i put it all back together.