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Sounds like that new thermostat got itself unstuck. I'd leave it alone but watch closely to see if it acts up again. It does, dump it. You can safely run without a thermostat, but it causes premature engine wear. Easier while you're in there to just throw a new one in. Most are good to go, but sometimes you get a bad one like you did. Sometimes they cure themselves and sometimes you have to reenter the patient. I've had 'em where they didn't make it out of the operating room, much less down the road. Always buy the best, that's usually the problem with new thermostats that fail. Think NAPA.