Because I’ve seen this multiple times before, and in every prior case it was always adware in some shape or form (the most recent was a proxy running on localhost that injected ads into some pages).
If it’s only shown outside of the US, then it may explain why I cannot reproduce this. A network log from page load onwards when this issue occurs would be helpful.
So if you pause AdBlock completely, is it reproducible?
Here’s a modified test version with script access disabled. Is it reproducible here?
http://nightdev.com/testing/noscriptembed.html