Overheating

About a week ago, was going down the highway and the truck suddenly overheated. Since then I have done the following: changed thermostat which was sticking, flushed the system, new reservoir cap. New radiator as thermal scan did show some blockages in the old radiator. Pressure and vacuum test done to check for leaks, combustion gas check done. Water pump tested fine also. Now the vehicle will overheat every time. The only way to stop it from overheating is to run the heat which will control the temperature gauge and stop coolant from overflowing in the reservoir. That also tells me the heater core is fine as it puts out more heat than needed. Also have bled the system multiple times. No coolant leaks any place or anything coming out of the exhaust. Upper radiator hose always gets hot, but lower radiator hose is lukewarm at best even with the new radiator installed. Completely lost as to what the problem could be unless it is some type of sending unit however I am not getting any codes. Put a scan tool on it yesterday and let the vehicle get close to overheating. The scan tool read a temperature of 213 to 231°, however the gauge in the cluster ran close to redline almost the entire time except when I turned on the heat and it would lower back down to normal. Any suggestions would be greatly appreciated.