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Well, looking at your other topic I would say there might be a problem with your device. However, while charging, the phone is allowed to use more resources.
Your phone has water damage, contact your local xperia care as suggested on your prior post
http://talk.sonymobile.com/t5/Xperia-Z2/Xperia-z2-problems/m-p/1063823#M35089
Perhaps the water didn't reached the water damage indicators but did reached the motherboard, just get your phone inspected by your Local support