I tried the suggested filters, near flag, and browser ID.

But, results are the same as my previous attempts. The first 2 pages work
properly but then the formatting breaks.

Might it be an issue similar to the following?
<http://forum.httrack.com/readmsg/23023/index.html>

I still wonder if HTTtrack is struggling to work through the variable
length/structured pagination of the thread. I tried your suggestions in the
pagination thread, but that didn't help either.

Edit: Actually, I think this might have worked but in an unexpected way. The
"showthread.php" driven pages do break, but the "printthread.php" pages are
working just fine. HTTtrack appears to be able to spider that navigation just
fine without causing any breaks or formatting issues.