New info: On wunderground.com I can bring on a freeze on a 10 day view page by not doing anything for an indeterminate while.
And I can bring one on on an FTM Posting form by doing nothing for a similar indeterminate while, BUT only if my focus is NOT on the "Post:" box, i.e. no flashing cursor in the box.
This has been 100% reproducible so far.
Chances are the delay is why the bug is so obscure.