Glitch flickering across screen

So I've trawled through all kinds of mac discussions and threads and found nothing so I'm praying that by asking the question myself, someone can help me (pls).

So recently (since I upgraded to high sierra) I've noticed that whenever I'm watching a youtube video on google chrome, this thick, white (sometimes other colours mixed in), pixelated line flickered across the whole width of the screen very briefly, like half a second long. I thought it was just a problem with chrome so i downloaded firefox and the same thing happened on the very first youtube video i clicked. It only happened on youtube until i went to an online shopping website that has a large background graphic and it happened again. So far it has only happened on browsers and not on any other app or the desktop. I have reset the NVRAM and PRAM, done a diagnostic check which has come up fine, and done first aid on the disk, all to no avail. I even downloaded a well-rated antivirus program which found nothing. I also reset chrome and that didn't work either. It doesn't happen every day - I upgraded to high sierra maybe a month ago, and this problem has only started occurring for maybe a week or 2 max. The only things I am yet to try is uninstalling chrome and reinstalling, or trying to go back to sierra, but I don't know how to do that.

Pls send help am desperate