2002 Pathfinder overheating after about 20 minutes of car running

I have a 2002 Nissan Pathfinder. A couple of months ago the check engine light came on and the temperature gauge quickly hit the roof. I was able to pull it into my mechanics shop before any damages was done. They looked at it and said I needed a new thermostat and housing. A few weeks later the temp gauge again went up to near the top. I live, work and shop all in a close radius so am not in my car for long periods of time but the gauge seems to measure high after about 20 minutes of driving in the city (no more than 45 mph) with the AC on. I turned off my AC and took the car back. They then replaced the fan belt and clutch that engages the fan.<?xml:namespace prefix = o ns = “urn:schemas-microsoft-com:office:office” /><o:p></o:p>

Took the car home and two days later temperature was up again. Took it back and they said they had not realized my car had a second thermostat so they changed it (at no additional charge "since they didn't catch that up front") - they kept my car for 2 days. A couple of days later the scenario replayed itself. I took it back in they changed the sensor to the temp gauge (at no additional charge) tested it and still overheated so now they have now also flushed radioator 3 times for any debris. They have had my car for three additional days now.

Please help!!! Thank you and hope to hear back from you. Andrea

Does the radiator cooling fan run when the engine heats up?
You did not mention anything about your mechanic testing and/or eliminating the electrically-powered radiator fan as a cause of the problem.

At this point, due to the repeated overheating incidents, it is possible that a head gasket has been breached, or that other engine damage has taken place. I recommend that your mechanic perform a compression test and a cylinder leak-down test to assess whether there is a head gasket-related problem.

From The Horse's Mouth: Nissan Says Not To Overlook Inspecting The Radiator Cap On Overheating Nissans.

Some of the caps have been found to suffer from a case of swollen gasket. The rubber gasket that seals the cap is backed by a metal disc that is supposed to be slightly larger in diameter than the gasket when viewing the under-side of the removed cap. A rubber gasket that has swollen, extends beyond the metal disc so that the metal can no longer be seen. This can cause the cap to not vent properly and result in coolant loss and overheating, according to Nissan. Also, they recommend replacing any cap that has foreign material on the gasket or its retainer.

This is just one of many items that should be checked.