Temp in gauge changes erratically after warm up

Are you paying attention to the coolant reservoir?   You don't say anything about this.   If you keep the radiator cap on, any bubbles in your radiator should be expelled into the reservoir as the engine heats up, and when the engine cools off, fluid should be pulled into the radiator to keep the radiator full.   That's why you need to keep the reservoir at least one third full when the engine is cool.   The reservoir should automatically have more fluid in it when the engine is warm.

He is correct as stated but that is the final step.  Another trick to purging the cooling system is squeezing the upper radiator hose by hand while the engine is cold with the radiator cap open.  During this procedure, note any air bubbles coming out, topping off the radiator in the process, and taking care to minimize spillage.  This process is done when air bubbles are no longer surfacing.

Found the issue, connector on sensor which looked good, upon further inspection found plastic was holding wire which was barely making contact also further inspection found radiator cap had been changed to like 8 lbs so replaced with 16 lbs really holding pressure here in Vegas where it is very hot next job is to add a switch to fan relay to turn on when it hits 200 rather than wait until 225. Also found lose wire in undercarriage going to front fans.

What year is your car?