Has anyone had a problem with elevated operating temperature (100 to 105 degrees C) at highway speeds caused by a radiator with insufficient fluid flow?

My 83 300D runs about 100 to 105 degrees C with the a/c on at 75+ mph in 90 to 95 degree weather. I have a new thermostat and have flushed the system, checked both fans and fan clutch, and have no pressure in the hoses after cool down, but the radiator is original.

I'm thinking that the radiator coolant flow is being restricted. My research indicates the radiator should flow roughly 20 gallons per minute, and radiator shops frequently do that type of test.

I'm trying to think of a way to do the test myself, and will just buy another one if it can't flow roughly that amount. I was told by a radiator shop today that he can't "rod out" my radiator because he can't get the gaskets he needs to put the plastic tanks back on. I do seem to remember someone repairing my upper tank on my 87 300SDL and they were able to do it, so I might give them a call tomorrow.

thanks of any help.

Joe