In addition to the other advice you've received,
the pages are sometimes put into the site in encoded form.
Try searching the source code of your pages for the string "base64_decode".
When I click on the link, the page comes up and the URL still lists my site. However, I cannot find the page in the directory on my server OR in my database.
Since the pages don't exist in the directory, they're obviously being redirected somehow.
If the URL still shows your site, it may be that the requests are being rewritten (a different page served than the one requested, without informing the visitor's browser that this has been done) rather than redirected (which sends a message BACK to the visitor's browser to please request the different page).
The question is whether the bad pages are inside your site or whether visitors are being redirected. The circumstances seem to suggest that the bad pages are hidden in your site somewhere.
For further investigation, the Firefox add-on called Live HTTP Headers will show you in real time whether any actual redirects (301, 302) are occurring.