fasterhatch

i have an automatic 95 civic ex and it used to overheat all the time but recently whenever i would stop the idle would rev aggresively. i replaced the thermostat and the overheating and idle problem stopped, i even got my heat to work again. but i left my car running for about 15 mins and all of a sudden, the idle prob came back and it was overheated! i drove a couple of miles after i let it cool for an hour and it overheats again but not as fast.
if its not my thermostat, what is it?