i dont have much experience in the broken thermostat field so i was wondering what some characteristics of a bad thermostat were.

obviously overheating is the major one.

one thing i also noticed was when i take the radiator cap off and let it warm up for a while it will start to overflow slowly out of the radiator. then itll go down and the coolant will start surging about every 3 seconds. it will go down then it will shoot up like a fountain and it will repeatedly do this until the level just gets too low and the surging coolant doesnt reach the top of the radiator.

i was thinking it might be a blown headgasket but the car doesnt smoke at all and it runs perfect other than the overheating problem. and i know it doesnt necessarily have to smoke when the headgasket is blown but its usually a common symptom.

any ideas????