My 73 450SE and 75 450SLC both did the same thing. When is the last time you changed
the coolant? Moreover, drilling (8 ) 1/8 diameter holes somewhat equally spaced in the
thermostat body should help solve the problem. I received that tip from a M-B Tech.
Doing both of those things solved my over heating. Before I did that, I back flushed the radiator, but that did not solve the problem. Good Luck
Old Max