Q: Screen flickers for ages when first go to use iphone

I have an iphone 4s.

This issue has developed over time, it did not happen suddenly, it has gradually got worse and worse over a short period (a week or so)

I do not have a clue what caused it to happen

When I go to use my phone after it being locked it flickers like crazy, there are many black lines that move up and down the screen which gradually disappear and go to the picture on my home screen.

It flickers for a long period of time. At first it was quite a quick flicker and I could then type my pin in and use my phone as normal until i locked it again. However, it has got a lot worse since then and now it flickers so long that by the time it has stopped flickering, my screen has locked itself. I have developed a technique of tapping my screen so it knows i am responding to the screen so that when it has stopped flickering, it hasn't locked itself and i can use it as normal.

I know I could put it on no lock but I want it to lock sometimes. Its a right pain especially when somebody is ringing as i have to wait for my screen to stop flickering to see who is calling, sometimes they have hung up by then! Seems to me like an issue with the pixels on the screen. I just want to know if it can be fixed and if so, what needs to be fixed?