Hi @Roy73, sorry to hear about this. 
How often does this occur, is it on a daily basis or does it occur once per week?
When the screen turns off, can you instantly press the power button and type in your screen lock, or do you usually have to wait a while? I'm trying to figure out if the device restarts on it's own or if the screen just turns itself off.
A good place to start is to restart your device in safe mode and use the device as you would normally do, apart from having access to any downloaded applications as they are temporarily disabled in this mode.
Let me know how it goes! 
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