hang on dude, it doesn't sound like the thermostat.....sounds like just the radiator cap!
the spring in it may have lost its tension and as the pressure builds up its letting the pressure through to the tank......this is very common and i recommended to replace the cap yearly (i do it myself every year).
thats the purpose of the cap and surge tank.....when something is wrong in the system and pressure builds beyond its operating range, hence beyond the cap's spring tension, it "gives" to relieve pressure in the system.
and as the car stops and the system cools, it creates vacumn (from the loss of water) and sucks the water from the surge tank back into the radiator.
if you ever notice the water drying up in the surge tank, thats a sign you have a leak somewhere and the system is drawing water from the surge tank whenever it cools down......
if the thermostate fails, it either stays closed or stays opened...
if closed - overheating.
if open - runs cool all the time.
try the cap first - easier and cheap.