What Kevin said. Also, it's possible you have a seeping or blown head gasket. There is a hydrocarbon test you can get from Napa for about $20 that will tell you if there are exhaust gases in your coolant. Also, look for gunk in the overflow and white goo on the bottom of the oil cap. It's quite possible you simply have a leak that is letting air into the coolant, so a pressure test is an inexpensive test that should be done. There are other, more complicated and expensive tests that can be done, but the hydrocarbon and pressure test shouldn't cost more than $50-60 at a shop - or boo row the pressure tester from you local auto parts store and do it yourself. GL, fingers crossed.