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First thing that comes to mind is a loose or damaged gas cap. I would check that first. Im at work and dont have my service manuals. If that doesnt colve the issue, we will dig much deeper!
The reason why some of the issues dont pop right back up is that the PCM directs that that diagnostic check be preformed at different times and after certain conditions have been met ( ie. engine temp min 150 deg ).
Check the cap, examine the gasket, apply a little silicone dielectric grease to it and slap that baby back on!