Intermediant high temp on gauge

the temp gauge sends the power from the gauge to the sender unit which is a variable ground. That means that as the coolant heats up , the resistance in the sender is decreased and the current through the gage increases ans so the needle goes up
when that wire is shorting out some where the gauge will go to hot and when that short is removed the needle comes back down to cold
so that means that you will need to trace the wire along for where it is rubbed through or get an accredited auto electrician in to find the fault

Air pockets will cause the temp gauge to spike high, then go back down after a few seconds. Why do you think it is not circulating?

check the temp sensor may need cleaned of water or rust .take off the wire and clean it.

could be the wiring, faulty temp gauge in the dash. or bad temp reader inside the engine. located where the top hose enters the engine.

This is a sign that the thermostat is sticking shut and then opening after the temp gets past a certain point. If the heat gauge goes to high and then drops down suddenly, this is the likely culprit and you need to replace the thermostat - again.

I have had them arrive from the parts store and be defective.

If tranny fluid is clear and at normal level, then the issue is with the transmission - it is getting ready to go sayonara.

Better get that fix before you get stranded.

Gage failures are very common on this truck, more than likely the gage stepper motor that drives the gage needle is defective.

The car is not overheating if the temp gauge goes up and down by turning the lights on and off.  The gauge is giving you bad information.   You can confirm this by starting the car from cold, sitting in the driveway, let the temp go to somewhere BEFORE normal operating temp, then turn on headlights and fan blower on high, watch the gauge, then turn them off.   If the gauge moves significantly from the electrical load, you probably have a short or a bad ground connection.  

You have a short circuit or bad wiring somewhere in the front half of the car.  I don't know what thermostat switch you have replaced ... but it's not the problem .. stop buying them.  
If you are the original owner, was there an accident that may have damaged some wiring?  If you bought the car second hand,  get a carfax and find out where the thing was hit ... concentrate your wiring trouble shooting in that area.

I think it is something OTHER than the temperature sending unit or the gauge

intake leak or a bad idle air controler