Search form disappearing

Hi, I think this belongs here but it could be a Views issue, but I'll start here.

I've got a panel page here:
http://www.proequest.com/search/horses

And the left-hand side is an exposed form. Randomly the form (but not the results) disappears, unfortunately removing one of the major reasons people come to the site. Clearing the cache brings it back.

It was happening only once every week or so but traffic has picked up since the Olympics. When traffic is high, it occurs once per hour.

Caching is turned off for that item in the panel.

I saw the issue here:
Panel content disappears
http://drupal.org/node/1569568

which might be related. For this site, the custom panel pages are entirely within the database, not coming from modules i.e. they are "normal."

Any help or insight would be greatly appreciated.