Thanks for everyone's help. I checked all (almost) the suggestions,
such as clean the evaporator coils, check fans, etc. None worked.
Fortunately, the technician came today. The problem turns out a
bad overload relay that connect the compressor. We had many
power outages since Hurrican Isabel. The ceramic inside the
small relay was almost broken into pieces.
Cost $153 to fix (that relay cost $40).