First:
Does it happen in Safe Mode too?
Tip:
Go Youtube and type "Xperia Safe Mode"
Second:
Put your SIM card in someone elses phone to test if it is the SIM card or your device.
Ask again when you have tested and it is not the SIM card.
PS
Did you know that there is a Sony Xperia channel on Youtube?
Just type "Youtube Xperia Channel" in Google search.
Here is the link.
