After you've done what rollamods has said... To clean the radiator, you could go to a place to get it "reconditioned", which means they'll probably just flush it out and it'll unblock about 10% of the gunk from the core and about $200 of your wallet. A much better option is to get a completely new core or even a new radiator made from scratch, which should cost you $350 - $400. In my old corolla I first got my radiator "reconditioned" at one of the large radiator chains. It did nothing at all. Then I got a dual core radiator made from scratch at an independent and never had another problem. If it's the thermostat, genuine ones are reasonably cheap from Toyota, don't bother with cheaper options. Oh, and you need to buy a new gasket that matches the thermostat too.