i have the same problem before and this what i did. hope you fix it.
1)Locate the heater and A/C blower motor under the passenger side dash. Removing the dash may be necessary to access the blower motor on some vehicles.
2)Disconnect the blower motor electrical connector.
3)Remove the retaining screws to the blower motor assembly.
4)Remove the box and blower motor assembly.
5)Remove the blower motor retaining screws. Remove the motor from the box.
6)Compare the new blower motor to the old one to verify that you have the correct part. Install the new motor into the box and tighten the retaining screws. Re-install the blower motor assembly.
7)Connect the blower motor electrical connector.
Operate the heating and A/C system. Verify that the blower motor operates at all speeds and that the system produces hot and cold air.
It sounds like you have an air lock in heater core. This makes for steam in the cooling system, that is why your temperature gauge jumps around on you. When the thermostat got changed, the air was introduced to the system, and it can't get out.