Running cron breaks the primary tabs

I use two background images to create the primary tabs, the left and right backgrounds, similar to the way zen themes them out of the box. I noticed after every cron run, the right side disappears. (see attached images) After some researching, I discovered the span tag (the hook for the right background image) that zen inserts into the link is missing after every cron run. The main navigation is themed the same way (same background images, including the span tag) but they display fine. Clearing the cache fixes the problem. I have not noticed any errors in the log. It just says cron run completed. We have been having memory limit issues and I am working to get that fixed. Could that be causing a problem? Is there something else I should be looking for? Css and js optimization is not on. If I can give any more information that would be helpful, let me know.