MySQL is acting funny again, albeit in a less urgent fashion

[Cold Steel]

Please take a look at the following links:

[url]http://aforums.com/forumdisplay.php?forumid=15&daysprune=1000[/url]

[url]http://aforums.com/forumdisplay.php?forumid=15&daysprune=20[/url]

Pay special attention to the dates. In the first link, there are several threads missing.

Any ideas as to why this might happen? Any ideas how I can correct this?