It sounds like you might have air in your cooling system. I have had this happen on a few cars.
What engine are you running? If it is the LT1 there are air bleeder screws in the smaller radiator hose and on the thermostat housing. Run you car for a while and let it get up to operating temp then shut it off and let it cool then open the bleeder screws. If air comes out this should fix your problem, but if it doesn't and anti-freeze comes out you have another problem.
I was banging my head against the wall when it happened to me.