Well, we're getting somewhere.
The Wunderground tech gave me a free "
no ads" upgrade, because he had a feeling that something in the ads was at the root of my issue...causing a memory leak, and the issue has, in fact, been resolved (with the side benefit of fewer trackers
).
I suspect that the tech could not replicate my issue because while his legacy installation is running OS X 10.6.x it is also running Safari 4.0.3, which differs radically from Safari 5.x. I've tried to get him to upgrade his Safari installation (in a clone, at the least), and I'm waiting to see what happens.
Edit: I forgot to mention that the ads were apparently at the root of my issue despite the fact that I had AdBlock blocking them.