Air pockets usually create hot spots not cool ones as it cause lower internal pressures. So i wouldn;t think it would be air.
Does the engine feel hot ?
Do the rad pipes feel hot as in can you touch them after running the car for an hour or are they warm enough to handle holding them. Is the climate control in auto or are you manually selecting fan speed and temp?
Is the fan on the rad spinning at full speed?
If the car is running and the heat quits but the engine is still hot it is either the rad fan is stuck on or the control valve on the firewall has closed keeping the hot coolant from going into the evap core to be blown by the fan to heat the interior.
I doubt it is the valve as the temp needle dropped as well. So it might be the thermostat. How long does it take to heat up from a cold start?