You could ...
Do a rest/reboot by removing the back, remove the battery, make sure the SD Card and SIM cards are firmly and squally seated, replace the battery, replace the back, power on.
Turn off ... Settings | Display | In-pocket detection .
Does it have a case and/or screen protector installed. If yes ... try removing them.
Are you running 5.5.886 or 5.5.893? Upgrade to 5.5.893 if you haven't already.
... Thom