Unknown cause. My game has had this flickering over the last couple of days but I couldn’t figure out why. Just a couple of minutes ago I’ve locked it down to the UI. Using spells and abilities, equipping and un-equipping gear causes then entire screen to flicker. Rapidly changing gear reproduces the bug to a very noticeable effect. This can be reproduced with no addons as well as with addons. Drivers are all current, and addons are as up to date as they can be.
EDIT: Accidentally posted under my classic character profile. This is for Retail.