I second bullitproof's comment about the terminal block on the right fender. I have seen the connections there look great and be basically insulators. The black corrision gets to the connectors. Just unscrew the connections (one at a time) and make them shine again. I was pulling my hair out one time until I actually got out my meter and started checking voltages directly from the alternator.
Other thing I have found. When you get large temperature swings in a day (like in spring and fall). The battery terminals seem to get bad connections. They shoud shine too.