Check the temp gage sender on the engine for corrosion and a snug fit. Intermittent high resistance could cause gage flutter. Also, an air bubble in the cooling system could cause the gage to bounce, but most air bubbles will eventually purge after a few hours of operation, unless they are REALLY large volume from a poor coolant filling technique. The gage will only be accurate if it is covered with liquid.

IMO driving in a parade is about the worst possible type of service. Idling for two hours straight is just not good for a car.

I've driven all my cars on racetracks at nine-tenths, but I would NEVER, EVER even consider driving them in a parade!

Duke