I was traveling this past weekend in my 2005.5 2.0T Quattro (24k miles) and began to notice my temperature gauge acting differently. It has always warmed up to the center line (vertical) and stayed put while driving. It now will sometimes drop all the way back to the first line while driving. Especially while coating downhill and the engine isn't under much strain. It has not begun to overheat, just overcool.

I am guessing this relates to the thermostat valve remaining open too long hence allowing too much coolant to flow and dropping the temperature. It is under warranty, but I doubt at this point if I take it to the dealer they will spend enough time with my car to notice the problem. Should I attempt to get this fixed right away or is it safe for me to hold out a bit more for it to trip a code or at least be more noticable. My real concern is it will get stuck closed and I will overheat. Any thoughts on how to get the dealer to fix this and not just shoot it back to me until it turns into a real problem?

Over my 6 hour drive yesterday it certainly was happening regularly on the highway.