Apple Mail has been acting strangely in the last month or so. When I first turn the app on, mail loads properly, but after a few minutes the small triangle with exclamation point shows up. As soon as I click on Window / Activity, everything reverts to normal for a while, then the triangle shows up again. I just installed Mountain Lion yesterday thinking that would solve the problem, but no joy. I keep a clean, well-maintained machine, so I have few technical issues I haven't be able to fix, but this one has stumped me. (F.Y.I. Before posting this discussion, I checked all the Knowledge Bases and could find no specific answer.) Any help would be appreciated.
MacBook Pro (15-inch Late 2011), Mac OS X (10.7.2), Thunderbolt Display, OWC Externals