Bleed it with the little bleed screw on the top of the tank. #8 in the realoem drawing of the radiator.
Use a contact thermometer or one like the A/C repair folks use to rule out a bad guage or sending unit. I spent a day once chasing a bad guage by changing t-stats and water pump. Water pumps usually get loose in the bearing and leak or wobble if they're bad. Check the temp all over the place - radiator, hoses, cylinder head, all the little hoses, wherever. With the cap off the overflow tank there's usually a small stream of coolant at the top flowing into the overflow tank. Also open the heater core with the temp switch on the dash. Air sometimes gets trapped in the hoses, etc.