2004 Ford Explorer

Engine Cooling problem
2004 Ford Explorer 6 cyl Two Wheel Drive Automatic 113210 miles

My problem happens spiraticaly. Some days no problems and other days my car tends to overheat. I have replaced the Thermostat and had someone run a preasure test(checked O.K.). The only time I actually lost coolant was when it ran hot and coolant spewed from the edge of the overflow/radiator cap. After that I replaced the Thermostat and it hasn't lost coolant since. It still runs hot from time to time. It appears to happen after the car has been idling. When it starts to overheat we can turn the AC off and the temp gauge drops to almost normal. I was told it could be a head gasket issue so I used bars leak liquid copper, but I am still having the same issue. Any suggestions and thanks for your help.
August 17, 2010.

Does it ever overheat at highway speeds or is it in traffic or stopped?

It has happened both in traffic and on the highway.
Usually after we have been in slow moving traffic or idoling in park then getting on the highway or moving normally in traffic. When we turn the AC on the temp drops. It has never started running hot when stopped(so far). Any suggestions?

Aug 18, 2010.
The AC turns on the aux fan, so that is why it start to cool. Have you niticed if the other fan is running when the AC is off?