The problem is probably not your thermostat. When the lower intake manifold gasket starts to go, you will get higher than usual temperature readings.
I know that you had the parts replaced just a short time ago, but there is not a great deal of torque on the lower intake manifold gasket, especially at the ends of the manifold and intake gaskets will fail if not installed correctly. If you are still within warranty on the previous job, it is worth going back to get the job done right. If you are not, go somewhere else.