I missed the previous posts, but try checking your timing. I had similar problems with my toyota and my saab. After much frustration, I found that both had ignition timing off at idle. The saab was an 1985 so maybe your MB has the same type of vacuum advance.
Check the vacuum advance diaphrams to make sure they are working. If timing is retarded at idle, they will overheat no matter how good the radiator/thermostat/pump/etc.etc is.
Just my experience based on the last two gasoline powered autos I had. Both the failures occurred at 130 to 160kmiles.