Really make sure your clutch fan is working...... They can be a total pain in the rear to figure out. I had a vehicle that for years gave me the same over heating problems.. FOR YEARS!! I had to drive to the temp gauge.... I changed the radiator. I changed the water pump. I changed thermostats 6 times!! The fan always turned, "obviously", it was working. Then in the middle of a hot trip in Oregan blahhh the fan failed! Took a few hours to go a few miles, pour water on radiator drive a mile... step and repeat to a motel. Walked down the street got a new clutch, oem replacement.... put it in in the hotel...(they weren't pleased)... and HOLY MACKEREL the heating problems left FOREVER...

I have done nothing more in 87K miles and it never, EVER, exceeds the thermostat temperature. That included a heavy 6,750 pound 4WD Suburban loaded to the gills, 4 people, in Death Valley, in the summer.

Right there I learned that a spinning clutch fan is NOT necessarily a functional clutch fan. If I ever have an overheating problem in a vehicle with a clutch fan again I will replace the clutch first!

The clutch can fail, essentially limiting the horsepower to the fan. Instead of maybe 6HP running the fan you may be getting only 3HP. But no one can look at a fan and perceive that difference. But to the cooling system that is the same as covering half your radiator with insulation.