iPhone 5s constant crashing and closing apps

I've bought a new iphone 5s and i'm having A LOT of issues with it. Not only the TouchID failing all the time but also the crashes, which is my point here. Even if i'm just doing normal activities such as browsing the web, or looking apps on the App Store, it will just close the app and bring me to the home screen, but that's not all. Sometimes, when playing gamings(not heavy ones, the iPhone doesn't overheat at all) the entirely device freezes/crashes and I'm forced to wait 10 seconds for it to start rebooting.

I had an iPhone 5 before and this rarely happenned to me.(I think it happened twice in just 2 months). But this iPhone 5s crashes once a week, and it closes apps(no backgrounds apps running at all) almost daily.

I'm wondering if it's a RAM problem? I downloaded an app which checks the current % of RAM used, and it always tells me ~75% of the RAM is used, even if nothing is running on background.

I'd like to know if there is a solution that I can get through these forums or if I am supposed to take my device to repair, because I'm pretty sure this is not normal.