'06 4.0

Hopped in the truck today and started driving...got up to operating temperature and still had no heat. I noticed the temp gauge was jumping up higher and higher. I turned the heat off completely, and the temp dropped back down. As I drive the temp stays around normal, but then climbs back up until its high again, then drops back down. Anytime i turn the air on, it climbs back up to red. I am thinking it is a thermostat, but its weird that it goes high and then back to normal. Fan is spinning normal, don't think it's the fan clutch. Resevoir is filled to normal levels. Radiator has a small leak, but nothing catastrophic, although I will be replacing that soon.

Any thoughts/ideas/other things to check would be greatly appreciated.

Check the level in the radiator when its cold. You may find it's low. The reservoir relies on a vacuum to draw coolant back into the radiator. If there's a leak it can jjst suck air back in through the leaky spot instead of coolant from the reservoir.

If it is indeed full, I would suspect thermostat as you suggested.

Test or just replace the radiator cap. They are cheap. As mentioned above, it may be just sucking air back in as it cools. Listen for gurgling behind the dash. As the coolant gets low, you would hear air in the heater core.

When you notice the temp drops from normal to cool its because the coolant no longer reaches the temp sensor.