Sounds like you need to check the phone for viruses, malware and bad apps.
Start it up in "Safe Mode" (instructions below) and see if the problem happens there. I am betting it won't. If that is the case, there is definitely something on the phone that is causing the issue and you need to remove it to make it stop. Should you be unable to determine what the cause is, then you may have to consider a "Hard Reset" which would wipe the phone completely (meaning you would have to back up personal stuff first) so I would use that as a last resort.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.