1998 Dodge Stratus inconsistent heating

I've got a 98 dodge stratus that has some really inconsistent heating going on. its been blowing cold air but then once the car heats up it will blow hot air out of the heater, now i know that part of it is normal, but what throws me is after awhile it will start blowing cold air again. The car was overheating and the thermostat was replaced, and that took care of the over heating, but it still blowing cold air, and we've notice a very small leak somewhere on the passenger side of the car by the resivoir tank. When you shut the car off you can hear a gurggling sound over by the tank. The resivoir tank is full, and lately we've been adding anit-freeze on a pretty regular basis to the radiator, usually after we add anit-freeze we will get heat for a bit, but then it goes away and blows cold. I'm not seeing any puddles under the car, and the oil looks fine, no milky color, so any input you have would be awesome.