Pressure build up in my cooling system

I,d take the thermostat out,and toss it in a pot of boiling water on the stove first.From sitting it could be stuck.It,s a quick and easy test.Putting stop leak in,is just a band-aid.If the head gaskets were leaking try re-torquing if still leaking replace the head gaskets.You have already put stop leak in so this will cofuse you,cause it could start leaking again next week.My 2 cents.:clock: