I still can't reproduce this, but it is still happening.
...It happens for both Firefox and IE, and both are updated (Firefox=ver 22.214.171.124, and IE=current)
...Since my initial post, the PC got a virus infection and everything on it has been completely reinstalled.
...The pages I get are now blank; "view source" shows nothing at all.
...I have messed with my own user control panel and cannot find anything that will make any difference. Sometimes the first page of the "commuting bicycle pics" thread will show up--but none of the other pages, they are all blank. This seems to have something to do with how the "view thread" setting is. The first page will show up in linear mode, but not in threaded or hybrid mode.
...I have looked in Sun Java and the Firefox settings to see if there's anything there that might cause this, but I found nothing at all. And I don't have this problem with other message boards, but then, they don't have threads with so many posts either.