I had the same problem. Coming out of sleep, Playstore would pop up randomly with some game. Lookout and Avast hadn't flagged anything. Tried factory reset (major hassle) but same pop ups kept happening. I tried StopBadApp and after one search it found 2 music apps which it considered viruses / bad behaving apps. After I uninstalled the flagged apps the problems went away so I noted as such in Playstore reviews.
Hope that helps.
Hope that helps.