Xperia Z2 :: On-screen Buttons Getting Pressed During Calls

First post here. I just bought my z2 and I absolutely love it but am deeply disheartened and concerned by one problem I'm having which pretty much is the most important thing I need.
 
Almost every time I make a call - the mute, speaker, end call or other buttons get pressed while I'm talking. I've tried to make sure that the proximity sensor is covered by my ear and it still happens. I've also run the diagnostics and although they pass I've noticed that the vibration for the proximity sensor test does not always stay on if I put the phone up against my ear and move it around. Eventually the vibration turns off and some on screen buttons are pressed.