Download, install and run the following two apps.
This one will check for any malware that may be installed on your phone.
This one will check for any viruses that may be installed on your phone.
The only thing that I can suggest now is that you back up the phone using the phone's backup feature. Then after ensuring that the battery is fully charged perform a factory reset using the phone's factory reset feature. Be aware that performing a factory reset will erase all your user data and downloaded apps. It will restore the phone to a factory default condition. That is why it is important to do a backup first.
Once you have completed the factory reset, check the behaviour of the phone. If the problem still exists then most likely it is a hardware problem and you should get it fixed under warranty. If the problem has not re- appeared after the reset, (allow sufficient time to elapse until you are satisfied that it has really gone) then restore the phone with the backup you made earlier, using the phone's restore feature.
Basically the sequence is backup - reset- check - restore.
If the problem does not re surface then hopefully the reset has fixed it. If it has returned then most probably a downloaded app is causing the problem. The problem then is finding out which one. You will have to uninstall your downloaded apps one by one until you find the culprit.