I've seen all variants of screen flickers.
One can be due to some developer options being selected - show screen updates, show pointer etc.
Another can be due to hardware problems;
- if you've ever dropped your phone or other sort of physical shock it can cause a loose connection, unlucky, uncommon, but certainly possible.
- if you've ever left your phone out in the sun, this can cause heat stress to the connections to the screen itself, very very uncommon and you would see the screen is a bit hard to read after that amount of heat.
- another cause of screen flickering is what I call "caveman syndrome" where I see people tapping the screen quite hard, over time it compacts the layers of the capacitive touch membrane so "Gronk" (my universal cave dweller name) just taps harder as time goes by.
- total screen blackouts can be the backlight having a failure, if you get an extended blackout try getting a good LED torch and shining at an angle and see if you can make out what should be on the screen, if you can see something using the torch it's a safe bet your backlight is on the fritz.
Cheers,
Scott.