well, if worse comes to worse, i would replace your radiator, hoses, water pump & thermostat, but start out with thermo. or see if it is working by taking it out and putting it into a pot of boiling water to see if it opens properly, if that works then put it back (or replace it anyways) and flush your radiator. if that dosent solve your problem keep going down the list, next you have the water pump (Chevy's are notorious for having the water pump go bad). you can check to see if thats bad if the fan blade has too much play (while the engine is off) wiggle it back and forth and spin it around. if it has to much play, that is most likely the problem. good luck.