[Dave]

Have you asked around if it happens to others as well? It's possible that you have a bad plugin installed in your browser or that your PC is infected with adware.

https://www.malwarebytes.com/mwb-download/

Edit: the only reference to ads I see in the source code of any page is:

<div id="ad_global_below_navbar"><p align="center">
<script type="text/javascript"><!--
google_ad_client = "ca-pub-6569655825755267";
/* global below navbar */
google_ad_slot = "3485033727";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</p></div>

Seems more like an ad that is displayed through Google Ads is not being so nice.