When I had High temps, it was a misreading caused by my Temp Comp board (as mentioned on an earlier post).

Last time I did the Timing belt and all the front parts were out of the way, I used the Garden Hose and blasted water from the engine compartment outward. I got so much sand and garbage out of the fins ..I was surprised.
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'75 Jeep CJ5 345Hp ChevyPwr and two motorcycles: it wasn't Volvos safety , it was Longevity that sold me

If you've already replaced the thermostat ( a good start!) then make certain the radiator isn't plugged with dirt and leaves- shine a light through it to make sure. If it's clear, then you've probably just got faulty indication- replace the temp comp board, or clean it's contacts, or jumper pin 1-3 as described in other posts here.
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"

Another thing to look at, particularly if the temp guage reads both extreme highs and lows, is the temp compensation board located in the gauge cluster. This board if making a bad connection will indicate erroneously high temperatures.

Tycho Nightingale

Quick check. Next time it gets up by the red, pull over a pop the hold. Usually if an engine is really overheated, you can tell. Things under the hood are way hotter than usual. Look at the overflow tank - is it overflowing?

The system is designed to run at about 14psi above atmospheric pressure. This raises the boiling point of the coolant to around 250F degrees. If the overflow tank/cap are failing - pressure is low, coolant can boil. May have a weird effect on temp gauge reading.

Also, try the karate jab system - detailed in another earlier of my posts.

Good Luck,

Bob

:>)