Yes a reset will remove files and more.
You might want to try starting up the phone in "Safe Mode" and see if it does this same thing. You won't be able to access many apps or files in that mode, but if it works fine then, I would say there is something possibly malicious on the device. Bad app, virus, malware, etc. Should it work in "Safe Mode" then check the device well for those problems.
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...