404 Errors After Permalinks Set

I have a problem that started recently, and it occurs periodically. Every so often, I receive a 404 error when viewing the category pages. It's always fixed when I go to Dashboard>Settings>Permalinks and save the settings. However, a day or two later, this is broken again.

I have given the link to my site's categories a custom name. This was set in Dashboard>Settings>Permalinks.

Any idea why this is happening?

Thanks,
Chris