2002 Envoy overheating

2002 4X4 SLT, 106K miles. Began running 10-20 degrees above normal under moderate loads, replaced fan clutch & thermostat, no signifcant help. Went to the Dealer, who insisted the new fan clutch was bad, replaced it AGAIN, and put in the "ECM updates for heating". Much better, until yesterday, pulling a steep grade in first low 4wd, pulling a 4K pound trailer, temp steady for 20-30 minutes, then within 20 seconds pegged. shut down & checked for broken hoses, coolant was audibly boiling, no leaks, started up, hot warning, idled engine and temp came right down to thermostat. finished the climb without further trouble. I am afraid there is an intermittent in the wiring to the clutch? Ideas??? Thanks!

Do a complete reverse flush of the dexcrap and replace it... Could be clogged cooling system....