I think that you need to look at the guitar set up.

As the guitar is made of wood it will be effected by atmospheric moisture, expanding in the summer and contracting in the winter.

This means that your guitars set up will need to be tweaked twice a year. Changing strings probably allowed the wood to contract a little further, causing the fret buzz.

I posted the basics of guitar set up here:
http://www.guitartricks.com/lesson.php?input=8670

Hope this helps