When the vehicle is cold pull the radiator cap off making sure that the radiator is full of antifreeze start the vehicle up now if you see bubbles coming up from the antifreeze almost at a regular pace it may be a blown head gasket
YOU MUST PROPERLY BLEED THE COOLING SYSTEM TO ACHIEVE HEAT AT IDLE. I JUST DID MINE AND WORKS GREAT. BACK FLUSH HEATER CORE AND REMOVE UPPER MOST COOLANT HOSE I.E. SMALL HOSE TO TOP OF COOLANT RESEROVOIR/ EXPANSION CHAMBER AND USE PROPER FUNNEL ATTATCHMENT TO BLEED ALL AIR FROM SYSTEM
This is most definitely a sensor issue. I state that because of the drop in the graph. It looks as though the engine comes up to temperature, then the sensor stops putting out the correct temp reading. Most temperature sensors work on a resistance basis. As the temperature it is reading goes up, the resistance gets higher, and the computer computes this ...
Is the upper radiator hose hot to touch after you have been driving,if yes the reading is clearly wrong(60-80F is less than hand warm).
If the hose is cold your thermostat might be stuck open,so the reading might be correct.
You can test the temparature by turning on the heater while driving.
does it blow hot air if yes the temparature reading must be ...