Odd overheating problem!, Hoping for help from you gurus Options

[wilc4]

I have a 1996 GT Convertible. Runs absolutely fine, except on occasion (it has happened about three times in the last month) I look down and the temperature gauge is well higher than it usually is at. Usually it is stuck - like a rock - a little under halfway up the gauge. When it does this, I look down it is probably 60-70% up the gauge. All three times it has done this I have been stuck in traffic stopped with the A/C on.

Now, here is the weird part. I turn on the heater and immediately goes back to normal (which isn't a surprise since it adds in the extra heater radiator) but then I can immediately turn the heater off, turn back on the A/C and it is fine. Last time it did this I popped the hood prior to putting the heater on and the fan was going full speed, so that isn't it. Also, I let it sit for a while and the gauge didn't seem to be moving towards overheating - but was just running hotter than usually (within the normal range, but certainly abnormal considering the temperature in this thing has never previously gone over its usual spot)

My guess is that it is a failing thermostat and that somehow putting the heater on gets it to fully open and then things are normal. What do you all think? Any other ideas?