Does your phone have enough freespace to operate efficiently? With iOS 8 it is recommended that you have 15-20% of the storage space on the device free. For a 16 GB device that is 2.4-3.2 GB free. For a 32 GB device that is 4.8-6.4 GB free. For a 64 GB device that is 9.6-12.8 GB free.
The next thing to do is Restore your phone through iTunes.
If that doesn't solve the problem Make an appointment at an Apple Store to have your device examined by a technician. Or contact Apple Support. Be sure to back up your device before taking or sending it to Apple.