I'll tell you what I would do. When problems start to arise I like to do a complete over-haul of that system. Your having trouble with coolant coming out of the overflow so what I would do is a complete coolant flush, change the thermostat, change the cap, inspect the house (if they are the slightest bit swelled, change them too) and inpects and change any other coolant hoses. Usually when one thing begins to malfunction, something else related to it follows right behind it. It may sound a bit extreme but that's just the way I do things and I have never had any issues. Good luck man.
