missing coolant

My 97 escort requires about 3 cups of coolant added to the radiator every month. So far I have not been able to find the leak.
The dipstick does not show any coolant on it. There is no blast of white smoke when I start the car.The transmission dipstick is not turning milky. Now the bad news. The overflow tank shows air bubbles and the engine runs rough for a minute when cold.
Could this be anything other than a head gasket?

Larry