Lately, my cars been gettin pretty hot. When I first start driving, the temp is fine, but when I shut off the car and come back to it after about 45 minutes has elapsed, the temp shoots way up, then comes back down. As soon as I start it, the gage reads a really high temp. Then after driving around (with the heat full blast) it will fall back down to about halfway, which is fine, but it should be just below halfway. I havent had a problem like this before with the temp. I know my coolant level is fine. Could it be the thermostat? Help me out.