Heater doesn't work

I have a 1997 Saturn SL2 with 122,000 miles. I'm having heater problems. This has been ongoing for about a year. When my heater started blowing cool air, I told my mechanic about it and he just topped off the antifreeze, which didnt seem to solve the problem. Then he replaced the thermostat. It seemed to blow "warmer" air for a while, but now, it's a little complicated. I will turn the heater on when the engine is warm, and it blows warm air for a while out of the top air vents, but then the air gets cooler, and the air from the floor vents is always cool. Also, my airconditioner doesn't cool as well as I would like. Would the heater/airconditioner problems be related? My mechanic mentioned possible water pump problems, which he will be checking for in a few days. I have no leaks, and the antifreeze jug seems to retaining its liquid. Any solutions or suggestions?