What a pisser!
I went back and looked at the first post, and all I can ask is the obvious based on what I would try if I were you:
Is the engine really getting hot, as in open the hood and you can tell? Does the oil pressure drop?
Have you tried backflushing with a good flush the system to be sure there's no blockage?
Is the timing correct? Not only at idle but at 3000 RPM.
Have you tried running it with no thermostat, or with a 75 degree to see if the problem still exists? My mechanic swears that a 75 degree thermostat will prevent overheating in some cars.
Since this problem developed suddenly, my bet is a blockage somewhere.
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,