The temperate gauge on my 2006 Chrysler Pacifica seems to be acting strange. It will head to the regular medium temperate and then raise to the H, but almost instantly go back to the medium or fluctuate between the medium and the H. It doesn't appear to actually be overheating, there's no smoke and no other problems I've noticed with the car. What is wrong and how can I fix it?

Remove the temperature sending unit connector, spray it with WD-40. and reconnect. I've seen it solve these problems several times.

If the cooling system has been recently open, you may have trapped air in there. That will cause those types of fluctuations also.