I didn't mean to hijack the thread.
I found the source (OpenX ad server) and shut that down.
OP - this is what I found about this problem.
If you see code for an iframe with width=“0” and height=“0” in the source code of any page on your website, you have found an invisible iframe. Iframes are most commonly inserted at the very top or the very bottom of a web page’s source code. A good first place to check for iframes is before the initial tag that starts a web page’s standard code, or after the final that ends a page’s code.
I found this code in any file containing "index" in the file name and in any HTML files on the site. Delete it - problem solved.