Flickering screen

My iPhone has little coloured lines running through the display and when i unlock the phone it shows up my apps and everything as normal for roughly 2 seconds and then the display fades out to a black screen though while the screen is black i can still navigate across pages and get into apps etc. i just cannot see anything. The only way i can properly do it is unlock the phone and scroll over a page then it fades and lock it, unlock and quickly scroll again before it fades out again. The only time that it does not fade out is if i am in messages or in the calls section, which i have to get into while screen is black then lock and unlock again then it stays. If i am in any other app it will fade out almost instantly, could anyone please tell me what is wrong with my phone?