The Thermostat opens and closes with a spring, so it is possible that the spring gets gummed up or just loses tension. It is a relatively cheap part and easy to install, so I would recommend changing it out (do you know whent he thermostat was replaced last?). Usually your tem guage should remain in relatively the same place, it might read hotter if you are idling in traffic for long.

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I should also ask, when was the last time you flushed the system? Just adding new coolant might leave chunks of old crap behind.

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2006 Fx4 Level 2 with a 5spd, Superlifted on 33's.