Vertical lines in sleep mode only

Hi,

Recently I changed the screen of an iPhone 6. Everything seems fine except that when it goes into sleep by itself there are faint vertical lines, which disappear as soon as I press the power button. When I press the button again the screen appears and works fine and no lines when I put it to sleep by pressing the power button again. Would you say it is

a) software problem or

b) defective screen

c) defective connection/connector (but then it should do this all the time)

Regards

Alok