It may be your GFI. I had the same problem and it was solved after I replaced the GFI. Alton told me that that usually fail on, not off, but that wasn't the case in my situation. The little circuit board they have on the cheaper GFI's is made out of paper and moisture can make them fail. The commercial GFIs use a plastic circuit board that is less likely to fail because of moisture.

HTH,
Gary