Many things can cause eyestrain. Not having a proper prescription is one cause.
I'm not sure what "flickering" you are referring to though.
Do you get eyestrain watching a movie for the same duration on the SAME monitor?
For the XBOX ONE is there any difference in strain between a game that runs at 30FPS versus 60FPS?
(You can Google to see what frame rate each game runs at).
On PC if you have VSYNC OFF you can get screen tearing that produces a type of flickering so you can try with VSYNC ON instead. As per the link there's brightness, contrast (affected by light in room as well. too bright monitor in dark room can cause issues) etc.
For computer monitors there are a few other things worth noting:
Again though, there's a lot of different things, or combination of things that can cause eye strain.