It seems likely that a misbehaving app may be causing this. The challenge then is to identify the bad app and remove it. You can try either removing all apps one at a time, or perform the Factory data reset which will clear all apps and user data. When the system starts after the reset, you should disable the Google backup and restore options so apps are not automatically reloaded. You should then test for the problem without installing any apps. After completing that, gradually introduce apps back into the system one at a time, testing between each until the problem returns.