Nissan Repair: engine temperature, radiator fluid, engine temp

Question
Hi, the engine temp gauge on my 96 pathfinder (135,000 miles) is going into the "H" when driving for a short while (30 mins).  When I turn on the fan blower and heater, it then drops back to normal for a while, but I have to monitor the temp gauge b/c it'll go back to Hot.  Hot air does come out of the vents but only when the temp gauge is above normal.  When the gauge reads on the lower half of the scale (from cold to warm), there doesn't seem to be hot air coming out of the vent.  
I live in the NY area, so during the winter this hasnt been a problem, but I know I have to get this fixed.  Might it be the temperature sensor?

Answer
Change the thermostat with a new one from Nissan.  The aftermarket thermostats are not worth the money.  Also, be sure to get the factory gaskets for the thermostat.  Change the radiator fluid when you do the job with a 50/50 mix of antifreeze and distilled water.