The now defunct Clubprevia.com's creator, Lextreme, was used to working on Previas. He had worked on quite a few and mentioned that the most common culprit to headgasket failure was a clogged return cooling line. He had mentioned that the majority of Previas he had worked on had crud in the cooling line that ran from the reservoir back to the engine... causing insufficient coolant flow. Lex recommended cleaning out the return line every three years as a precautionary measure. Not sure how accurate he is but like Tim, he has a lot of experience with these vans. Sounds like you could only benefit from doing this. Then again, I would think that proper cooling system maintenance could prevent it this as well. Food for thought...