I would be suspecting a bad intake manifold gasket or a bad water pump; that is assuming you did service the coolant as needed and kept the radiator filled right. The Vortec engine doesn't use standard head gaskets but O rings instead and they rarely fail outright,but the heads can crack if overheated and cycled too much with excessive heat. Inspect the water pump for seepage, there is a hole in the pump housing for most vehicles that drains coolant if it leaks. I would do a pressure test then, that will determine if the coolant system is intact or not. Not very often, the thermostat can fail, but that is a very simple mechanism. And yeah, if you lost coolant and have air in the system then you could have same issues, read up on correct process for filling the radiator if you suspect the coolant level has been low.