Ford Explorer

I have a 1994 Ford Explorer with a V6 engine. It has approx 168K on it. The Heater quit blowing hot air so I changed the thermostat and it fixed the problem for about 3 days. Thee heater was working fine until I turned the heater to the off position for a little bit while driving. When I turned it back on it started blowing cool air again. The temp guage wasn't in the normal position and I noticed it falling towards C and then back up again it did this several times while driving at highway speed. A day later it started working again. I turned it off again to see if it would do the same and it did. Not sure what to check next. I have noticed some other posts regarding the heater control valve and such. All suggestions would be greatly appreciated.
February 5, 2007.

Make sure that your raidiator if full of coolant. I have had the problem where the heater stopped heating and it turned out that we were just low on coolant.

Feb 9, 2007.