I would bet there is something wrong with your cleat.
You say you rode a few hundred miles before you started having trouble.
If you've already checked everything yourself, the next step is probably to take the pedals and cleats into the LBS and ask them for help. Frogs are notorious for releasing easily (for some folks, too easily). I can clip on my empty shoe, and just from gravity it will unclip itself when I let go of it.
If a pedal system starts misbehaving after a few hundred miles, chances are something has gone out of whack with the pedal or the cleat.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson