2007 DTS Not Starting Sometimes

I have a 2007 DTS. This is the 2nd time this has happened. I turned the car off and
pumped gas. Got back in the car and it didn't want to turn over as if the battery was
dying. I didn't leave anything on and the keys were in my pocket. I got a boost and it
started instantly. The other time it happened I went into the house came back got in
and it didn't start. Again a boost made it turn over instantly. In between times the car
runs perfect. I took it in to have the battery checked and they said the battery was fine
pushing over 13 volts. So, I'm in hopes that someone will have an answer for me
because I'm clueless as to why this is happening.

On another note, can somebody inform me to what is the wire that's near the
battery that has a little ring attached that's supposed to go on the screw that's
coming from the body of the car. When I got the battery checked we noticed it was dangling loose so slid it over the long screw and capped it with a nut. He had no idea what the wire was for. All I can say is it's a battery wire that we assumed went
over the tall black screw that's mounted on the body of the car. Maybe it's some
of ground wire or memory wire? Please help