I don't have any insight on fixing it other than doing what you've already done - the thermostat.. Could try to get all the air out of the system...

To further diagnose you could measure the resistances at the temp sender for the gauge and the temp sender for the ECU at a known temperature, say once you're running and you're in the middle of the normal range. That temp is 190-200 degrees or so...

Next time it starts that crap, shut it down and measure the resistances at both sensor.. I bet it'll show an odd reading at the gauge temp sender (which is higher in the motor) - ECU sender will probably read right where it should..

On the 3.0 I did see an odd case where the engine temp would get hot and then suddenly cool off.. that one ended up being a going-bad water pump (it would seize, stop, and then restart) - but it doesn't apply to the 22r series.