My samsung Captivate randomly shut off on me once and went blank once early symptoms of later destruction?

I just have had this phone for maybe a week and I'm a perfectionist so its in mint condition but i noticed it seems as if i download too much or do too much at once it has reacted weird only twice once it just restarted and the second time it just went black for a little than wouldn't let me use it for a little than the screen worked again. i have a few days to return it for another one or a different phone any opinions would be seriously appreciated!