iPhone 6: HELP! On/off button not working, diagonal line across screen

iPhone 6. 64GB. newly bought. i.os.s8.3

I have 3 problems

1) My Iphone 6 has a faint line from the bottom left of my screen to the top right. It doesnt occlude any pixel, everything in that line is just one shade lighter

2) My iphone 6 on/off button is a little wonky, sometime it works, sometimes it does not

3) WORSE OF ALL! MY IPHONE 6 CANT TURN OFF. everytime i turn it off, it will automatically turn on. It doesnt just turn on, it goes onto the apple screen, then becomes black then apply screen again, at least 4 -5 times before staying on.