I got my Captivate 4 days ago and I was really loving up until a day or so ago. The phone keeps shutting down by itself. It has had almost a full battery and it just shuts off. This is quite irritating and I'm not sure what the issue is..
Mine only did something like that after an app crashed. The screen went black, and I got no response from the phone. Had to pull the battery to get it to restart. But that only happened once.
The first thing I would do is think about what apps you have installed. Did this start after installing something? Even though the app in question might not be something you are starting up, it may still be loading into memory and causing problems. If anything comes to mind, uninstall it and see if that helps. If not, you can always try erasing all data from the phone and restoring it to factory defaults. That option can be found under Settings, Privacy.
If all that fails, you may have just gotten a dud.
I don't even have that feature turned on. No applications are running except a the ones for the system. It did it again this morning and it is getting quite old. I can't have it turning off when I am using it.. It has frozen on me before as well. I hate to reset everything since it took me forever to get all of my music onto it.
I plan on taking it back. It shut down on me 3 more times today. I don't think that I am going to get a replacement. I'm going away to school this week and I won't have time to mess with this issue again.