It appears that you are going piecemeal with the cooling system. All have to be replaced on your car that is almost 30 years old. Here is the system: water pump, radiator, thermostat, all hoses, overflow tank and coolant. I owned a 190 and live in the desert down the valley from Ismalley. We have 118F summers for 3-4 months. While the 190 didn't run the coolest, I never had a problem with the cooling system.
And I would still replace the coil to cure your engine problem.
Anziani
'97 CL600 56K