Overheating, at least according to the gauge and warning light.

What is your question?

Thus far, we have ruled out a stuck thermostat, a busted hose, a busted radiator, the fan relay switch; and we replaced the sending unit. At first, we thought we were getting a false warning, but turning off the air and turning on the heat caused the temp to go back to normal on the gauge. Heat works as does air. Both cooling fans engage, although one spins faster than the other. When the engine is cool, there is plenty of fluid in the reserve tank as well as in the radiator. Any suggestions as to what else might be checked or what else might be the problem?