Yup, pull the negative terminal off and bang it against something and see if any corrosion comes out, my
Jeep Grand Cherokee (which was having similar symptoms) was corroding underneath the wire. It looked clean to the naked eye, but it was rotting about 6" back and I had to splice it.
The easiest way to see if the wire corrosion is your problem (assuming the wire isn't rotted completely to begin with) is disconnect it....shake it, reconnect it and see if it starts again. Chances are it will. Then it will fail again after a few weeks or days.