Puk'in Fluid

Or just drill the thermostat and never have to mess with trapped air again.

It would be nice if we didn't have to guess, but it's all part of the diagnostics sometimes.

The VERY next step I would take is to get a cooling system tester on it and see if it is a compression leak making bubbles in the coolant.
There is a fine line in the diagnostics between overheating and pushing coolant out or pushing coolant out and overheating.