Let's get back to the basic question-- do you have an auto bleed line from thermostat housing to reservoir or other higher point in the system?
I now our old 3116 did not, but the coolant line to the air compressor came off the thermostat housing and was an easy place to crack open the connection and bleed until bubble free coolant came out.
Once you have proved that there is no air bubble, and problem persists, I would pull the thermostats and check them in a pan of water on the stove with a candy thermometer. Make sure the thermostats are properly seated and installed right side up!
If those prove OK, suspect a gauge issue.
