I had the same issue.
Noticed it in two modes.
1.) While scrolling through and displaying images. (Turning off Settings / Photos / View Full HDR seemed to fix this issue. Not 100% sure, however, because I subsequently did a hard reset of the phone and many, many other things that have been recommended and, now, I can turn HDR back on and that problem does not comeback.)
2) Still had the jittery flickering screen problem when viewing a website with Safari and any Ad was being displayed. Played with toggle certain Safari settings and sometimes (seems buggy and inconsistent) toggling Settings / Safari / Prevent Cross Site Tracking off and refreshing the website put Safari into a mode, where the Ad pics went away and, therefore, the flickering stopped. Noticed, however, that this solution (again seems buggy) because the results are not consistent. Sometimes the ads go away and sometimes they do not. Sometimes turning that feature back on, seems to temporarily fix the problem. Anyway, just seeing the association of the flickering be related to the ad pics being displayed, caused me to buy Ad Block Pro for $10 and that also solves the problem, because Ads are no longer displayed. Also, my phone seems to be blazing fast now with that Ap installed. Not sure exactly why, but I suppose it is preventing some data flowing between the web and my phone. Who knows, but I am very happy now!! Thought there may actually be some hardware issue with the phone!
There are free Ad Blocker apps that may help too, but I was so frustrated with poor battery life on this new phonej (seems I may have resolved that too!) and the flickering that I just went straight for the $10 pro version and life seems to be very good now! Time is $$$ too!