When you download certain Android apps from the Google Play app store, they bundle in Vungle and others. Start over with a factory reset and add one at a time till you find the app with the adware.
Just when the tablet is idle, with or without any browser or app open, annoying adds randomly pop up and colonize the screen. For a few seconds you have to wait until a closing "X" tab appears. These adds are from "Vungle" and "HRS". I have checked and ensured that my web browsers (native plus chrome) do not allow pop-ups - so the issue is not pop ups from browsing.