Hi Super, Welcome to the forum. I doubt if your thermostat is the cause of the dancing temp gauge. A thermostat is simply a dam that restricts coolant flow during warmup and thereby redirects the flow to the block and heater core passages only, avoiding the rad. When the vehicle first reaches operating temp, the stat opens and is out of play for the rest of the drive. The first thing I'd check is the coolant level, if it is slightly low it would account for your temp peaking high during AC use. It may be time to flush and replace the coolant. The worse case scenario is that your Nissan is experiencing the onset of head gasket failure. If the symptoms persist after a flush and refill and no external leaks are found you may need to have some testing done to determine if the head gasket is on the way out. Don't forget to check the radiator fins, if they get blocked with accumulated debris it affects the heat exchange capabilities of the unit. Also check to see if your electric rad cooling fan is coming on. It should come on during AC use regardless of eng temp and should operate with the AC off when the eng temp peaks.
