On certain pages -- with no pattern I can detect -- a comma (,)
instead appears as (�).

If that last bracketed item does not appear on your display, it is a
rectangular box containing 4 letters on 2 lines. On top are the
letters FF. On the bottom are the letters FD.

I have checked my character set and it is what I presume is the
default -- Western ISO8859-15.

Does anyone know why this occurs and what can be done to fix it?
Thanks in advance.