Water pump and thermostat installed August 2011. First time this problem happened was June 2012, 8000 miles later. I have not had to add coolant. The first time it happened we were on our way home from Maine to NC and it only happened one time on that trip. Next time was when we left home going to the Outer Banks, NC, 380 miles one way. Didn't happen again on that trip. We can't remember the other times but it was always after an overnight or more sit. Sometimes this car may set for a week or more and not do it. The only thing in common is it is always early morning before 8 AM and on an interstate or highway within 15 min or so. I believe an air pocket (a very good idea) would have worked it's way out by now. I am starting to think it could be a gauge problem but it doesn't act like one. I have never looked at the gauge just before it happens, but the temperature drops over a period of 5 to 10 seconds, not instantly. I have an over temp, low oil pressure, low or high voltage, low fuel level alarm that gets my attention.

Paul

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84 500 SEL (307,xxx miles)