2007 PT cruiser, its been periodically overheating. I noticed a stream of coolant draining away from the car the first time this happened so I lifted the hood and found that the leak was coming from intense bubbling in the overflow tank. It'll get to the point where the bubbling raises to the top of the tank and drains out of the overflow tanks drain tube. This seems to only happen if I aggressively drive the car or maintain highway speeds for an extended period of time. The temperature gauge will slowly raise higher after slowing to a complete idle, then if sitting idle for a long duration it raises rather quickly. If I continue driving, the gauge drops back down to normal levels at the halfway line. Also noticed if I turn on the air conditioner it seems to make the gauge raise as well.. However, if I drive no faster than 40-50mph, it doesnt seem to ever overheat nor are there ever any signs of coolant leaks... I'm going to replace the thermostat within a day or two and hope that resolves the issue but I was wondering if there was anything else I might need to take a look at too for possible cause??? Any suggestions? Thank you for any advice ahead of time!