How can I backup if the backup always gets stopped and the iPhone 4S restarts itself (iOS 8.1.3 12BA466)?`

When I try to do a backup it always starts itself new. My photos and notes are synced but all the other apps not anymore since December. Plus since this weekend it refuses to go on the internet. Since January it restarts itself about 20 times a day. Music takes 30 seconds to loud. And all this despite this was a replacement model from the AppleShop bought just 1.5 years ago. I