Good batter, and from I can tell its properly installed. Sometimes it doesn't start unless i wiggle the cable.

I have a 2003 Nissan Xterra. I take care of it very well. For the last 4 month, my car will randomly not start, as if the batter died. When I wiggle the battery cables and try again...the car starts. The terminals are clean and tightly secured. What could be causing it? It happens randomly.I've had my batter tested and its good.