I have intermittent heat problems on my 1998 Ford Taurus?

The problem is that you have a vac leak at your selector switch or between that and the heater core. When you select where you want the heat to come from your losing vac because of a disconnected line or hole in it or defective switch. Try to listen under the dash and see if you can hear a hissing sound and try and follow it to determine where to look for the leak. The engine needs to be running to do this. While listening if you cant hear it try and move the selector switch from floor to defrost and see if you hear it.