I have a 2014 5.0 that had the water pump explode. I did the pump new thermostat and new belts. I have used a vacuum bleeder 3 times to make sure the air is out of the system and the temp gauge still fluctuates. Usually after 10 mins of drive and then shutting the truck off running into the store come back out and start heading home trucks starts to over heat. No codes

 

Are you sure there is no debris in there somewhere? What brand pump? What brand thermostat? You don't need a vacuum bleeder on this system. Just drive it a few days.

 

You may have a bad thermostat. When I changed my water pump, I changed the thermostat with a new OEM thermostat and the temp gauge didn't just sit in the normal spot. I swapped the thermostat out with another new OEM and it has been fine since.

 

Unless you used a Motorcraft thermostat, this is the most likely cause, be wary of driving further, this is actually overheating.