Is your iPhone up to date? The latest software for your device is 9.2.
Resetting all settings might help as well. You'll find this in Settings - General - Reset. This will set back your settings to default (also Wi-Fi, e.g.), so you'll have to go through the Settings app, and select/set up everything again.
If your problem persists, try restoring it using iTunes. Before restoring, make sure there's a most recent backup. After restoring, it's possible to restore from that backup.
If that doesn't help, set it up as new.
The last option would be to take your device to the next Apple Retail store, and let them have a look at your iPhone.