Sounds like you could have something on the device that is the problem. Virus, malware, bad app, etc. It could also be the OS or hardware, but start with checking for the other first.
See if you can start the phone up in "Safe Mode" (instructions below). In there some things don't work, many apps won't work, but you can test/use some things to see if the problem pops up at all while in there. If nothing happens, no problems, while in "Safe Mode" then you know there is something messing with the device (virus, malware, bad app, etc.). However, if it happens even in "Safe Mode" then you are looking at an OS issue or hardware.
If it is fine in "Safe Mode" then you need to uninstall any new apps, any recent updates and check the device for malware as well as viruses (they aren't the same).
If it is problematic in "Safe Mode" then you will need to consider either doing a "Hard Reset" or to have it looked at for hardware issues.
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.
To do a "Hard Reset"...
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option.
6. When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
7. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
8. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
9. Then press the "Power" button to hard reset the phone.
10. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.