Hey y'all

I have had my '03 Mustang for a little over a year now, and it has approximately 158,000 miles on it. Every few weeks my car won't start because there isn't enough power. Though, once I take the terminal clamps off and put them back on my car starts up fine. Before I realized removing the clamps and putting them back on worked, I replaced the alternator and had both the alternator and battery tested. The test showed no problems. As far as that goes it seems like I'm just resetting the computer, but I have no idea how to fix it. Along with that issue, sometimes when I apply the brake the lights on my dashboard dim and the rpms go just below 500. Does anyone have any idea what the problem may be? I took it to an O'Reilly's and they have no idea. In order to save money, I'm also trying to avoid bringing it to the shop.