A number of filemaker users have encountered trouble after an upgrade to snow leopard. Most often, the cause was traced back to an installed font that older versions of the Mac OS didn't have any problems with, but Snow Leopard does. You can search this forum using Snow Leopard and "font" to find a number of threads on this subject.
Phil. Thank you! I found a copy of the Arial font in the User's /Library/Fonts folder, moved it to the Desktop, reopened FMP and now everything is displaying as normal.
I found as well that the single record that was displaying (when the issue was ocurring) had the font set to Helvetica, and all "invisible" data was set to Arial.
Again, thanks very much for your quick response & assistance.