I'm thinking I need a thermostat

[sparky34kv]

I would check fluid level first and foremost, if it's low it could give you a temperature fluctuation when the system loses fluid flow. If you are doing the thermostat I would recommend replacing the entire manifold and thermostat housing. Dorman makes an aftermarket that is all aluminum and includes the thermostat. Since the water manifold is plastic, it does not take much for it to go bad, especially since the thermostat on our 2010 is 221 degrees! If you plan it ahead of time you can get it for under $100.