Weird indeed, especially if you have different ones each time
To be sure it comes from an ad, and to be able to report it, when it occurs can you check what is the "campaign id" of the ad?
cf https://www.sovrn.com/wp-content/uploads/2015/08/How_to_locate_CampaignID.png
and http://imcdb.opencommunity.be/forum_topic-8103-65841-Testing_a_new_ad_company.html#p65841 where we previously had ads problems