2000 ranger

i just did head gaskets in my dads 2000 ranger 4.0.....it had an exhaust leak for a while....which was just and exhaust manifold to head gasket....replaced that...now its got a problem where it blows the overflow cap off...and blows coolant out....not the radiator cap....but the overflow tank cap...i put a thermostat in it, just because it had a tendency to take a while to heat up...that problem fixed....but theres still the other problem...anybody got any ideas? thanks

seems like you are getting combustion pressure in your system. I would check for gases in cooling system. you might have to redo head gaskets. but check the head and deck for flatness first.