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Check the forum. Lots of other threads discussing the same issue and a post about a new Plex Media Server released to Beta today that will hopefully resolve the problem.
FYI, if you have the option to enable more frequent guide updates I would personally recommend turning that option off unless you are running a XML TV feed for your guide data. Plex from what I understand only gets guide data updates from Gracenote on a daily basis. The one time that I saw this happen on my system was when a guide update was running at 5:00 pm and removed a show scheduled to record that night.
