Q7 4.2 coolent overflowing out of the pressure tank
 

My 2007 Q7 4.2 started to overheat to 3/4 off & on for couple of weeks and then it went all the way to red. So I order and change out the thermostat this past weekend. All seem to be good, it drives great, didn't over heat. But, after stopping the pressure tank overflow started pushing out coolant. So I tried to burp the system and it didn't work. So I flushed the system and burp it again. But I'm still having the same problem. Water/coolant is still coming out of the pressure tank overflow after stop the engine. Car is running great and heater works great. It not over heating. Please does anyone have a solution for problem? It's driving me nuts! Trying not to take it into a shop. Thanks