At the end of my rope with this car. Here's the shortest version of the story:

Car had a head gasket job performed by a non-MB specialist before I got it due to blown HG from over-heating. These geniuses could not get the car to run at proper temp when they were through, so they bypassed the entire heater system via one hose from head-to-head in rear of engine. They told the customer, (my friend), that there was a problem with the heat system that would cause the car to overheat and that it would cost $thousands to fix. He accepted this.

He sold me the car as-is for next to nothing. It was his late father's car and only had 65k original miles. I replaced the radiator and thermostat + the H2o pump, all with good OEM parts. (I was admittedly grasping at straws with the pump). It ran slightly cooler with new radiator, then started running hot again in fairly short order. There is no mystery that there is a blockage somewhere. The heat is now connected, monovalves seem to work fine.

Before I take the entire top end off this car, anyone have a guess about something else? Here is what has been eliminated:

Fan/fan clutch
auxiliary fan
thermostat/H2o pump
Radiator is new

The car runs at least 20 deg. hotter than normal in traffic, i.e. 100+ on a cool day. I'm about to remove heads, what might I find? A plugged water jacket? The wrong head gasket set used, blocking a water passage?

The gauge and sending unit work fine. The temp readings on the dash are accurate. Thanks a lot in advance!