My recently bought galaxy s9 has horizontal strips of pixels that flash randomly across the screen (looks like multicoloured static). This happens about once a week and lasts for roughly one day. These strips don't show up on screenshots.
Next time it happens run a quick test and let me know if it appears.
Go over to your dial pad and type in *#0*#, this will bring up a number of tests. Try clicking on the 'Blue', 'Red', 'Green' and ‘Black’ options, and check if the issues are still there.
The strips have been flashing for the past 24 hours. The completely disappeared when i did the red, green and blue tests, however they still appeared on the black test. This unfortunately did not fix the issue either.