Sounds a tad fishy to me. Try starting it up in "Safe Mode" and see if the problem happens then. If it doesn't, then you likely have something on the phone that is causing this. Bad app, virus, malware, etc. However, if it happens in "Safe Mode" as well, then either you would need to "Hard Reset" the phone, which will wipe it and start it over as it was when you got it (devoid of anything you put on it) or have it looked at by a tech.
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.