Air in the cooling system will cause the temperature to fluctuate....and even overheat.
The air could be in there from when the head gasket work was done....
If there is a leak in the cooling system.....it will enable air to enter....and cause temperature issues.
There ARE other places for air to enter......lower intake manifold gaskets, loose hose clamp, etc.
By the way, were NEW lower intake manifold gaskets installed?
They are NOT re-usable.....so they MUST be replaced during the head gasket job....as you have to remove the lower intake manifold to remove the heads.
It sounds like you may have a electrical issue......which could be the battery and/or alternator.....as well as dirty/loose connections.
I would start at the battery, if unsure....replace it.
Remove and clean the inside of the battery terminal cables as well as the surfaces of the battery terminals....they can build up oxidation.....looking good, but not making a good connection.
Sitting around is a prime situation for bad electrical connections as well as draining the battery.
You have 2 temperature sensors, 1 for the temperature gauge, and 1 for the PCM.
I show them in one of my photos, in the pictures that the link in my signature takes you to.
They sit right next to each other.....above the thermostat that you changed.
I do not think that they are interchangable....as they are listed as 2 separate, and different parts in the part ordering information that I have seen.