This is a problem with the proximity sensor not recognizing your face being near it.
Do me a favor, and type *#0*# into the dial pad, and select '''Sensor'''
Run your hand across the top of the screen, and if the device turns green, and begins to vibrate, then your sensor is working fine, and there is a setting somewhere that is being missed.
if it does not, you have a failing sensor, and you need to repair it.