Remove and clean EVERY connection on both the hot and ground sides. "It looks good" isn't sufficient. Until they're all clean and tight every connection must be suspected.
I am with Jerry on this one. You would be amazed what kind of corrosion and yuck can get in those connectors under normal operation. They still *look* fine from the outside, but the vibration works it just a hair loose and some dirt/water gets in and just stays there, slowly making the connection worse and worse.
It doesn't take long to remove, inspect, clean, and reinstall those wires.
