1996 GMC Yukon overheating (gauge reads in red)

This was an intermittant problem but now is happening every time runs. Gauge indicates overheating, check gauge light comes on after only 6 or 8 miles. When slow down the gauge will decrease to normal in less than one minute, but goes back up if speed increased again.
radiator and water pump replaced about 2 years ago, changed to the "red coolant" which turned into brown goo and had to be replaced (went back to old green coolant) and the whole system flushed, neglected to flush tubing between resivoir and radiator, so had to add fluid/coolant to radiator (gurgling). Since Feb of this year we have flushed radiator, replaced thermostat, replaced the fan cluch, replaced temp cooling sensor and replaced the instrument cluster, with no improvement. I may be in denial due to the milage, but have taken very good care of mechanical with regular service and care at a GMC certified dealership which now has lost dealer but still has the certified mechanics.