Maybe your Tstat is upside down? Or you have and air lock in the system, if your sure that the Tstat is installed properly. then when the engine cold top up your rad, top up your reservoir (make sure the overflow tube in the reseviour in emerged in coolant. otherwise you'll be sucking up air when the engine cools back down) to cold level. Run the engine. there might be a air bleed screw at the highest point of the cooling system. Unscrew it until you see only coolant with no bubbles, close the valve and take it for a rip. should be at normal operating temp.

no bleed valve on this system. it sounds like you may have damaged the head gaskets from overheating. the t stat may have been bad but the head gaskets may have failed during the overheating. have a leak down test done to confirm.

Roy