1996 Chrysler LHS Sever Overheating

My car was running fine with no problems and then all of a sudden it over heated. We replaced the thermostat and had a new water pump put in about a month ago and thought that was the problem it fix for about 3 weeks and then back to overheating again. I drove the car approx 10 miles down the road after it ran in the yard for 25-30 min. Going down the road I noticed the heater would blow out hot air and then it would blow cold and then back to hot air. The check engine light hasnt come on at all and neither does the coolent light, but the gauge goes up to hot very fast. We have not drove the car since then. When the car overheats the water comes from the overflow for the radiator. My engine size is a 3.5 ltr.