Electrical problems

My 2006 Mustang (v6) wont start. It started about 4 days ago, I didnt drive it for a few days got in it and it wouldnt start. It just made a clicking sound like battery was dead. I jumped it off and it started fine. I took it to autozone to have the battery and altenator tested. They checked out fine. Took it home, turned it off and tried to start it. It wouldnt start, had to jump it off again. Also when im driving I noticed that when I start braking that my headlights and dash lights dim a flicker a little bit. My dash has a display that tells me how many miles til im out of fuel, and now it is acitng up. I was driving earlier and I know it had at least half a tank and it said 0 miles to empty. Then it went to 60, then to 5 then to 200. Not sure whats going on with this thing. Thanks!