You've tried and haven't been able to find the source of your problem. I've had a good experience with MyJoomla site monitoring so far and my security woes are eliminated. I've been "hack free" since I signed up.
Rather than wasting further valuable time attempting to track this prolem down, try a free audit at the site below and see what they come up with. It can only help but it sure won't hurt anything. Link:
myjoomla.com/site/is/hacked
It looks like the traditional injection via an extension that allows uploads (often the 3rd party editor) and you may find they have added the code to the template index.php
The most common exploit is the upload of an infected gif file so I disable gifs form sites whenever possible.
Can you use the 'secure' option above post input to provide me with a temporary Super Admin access please and I will take a look to see what the cause may be.
Thanks
Please search forums before posting. Please make sure your post includes the version of the CMS you are using and a link to the problem. Annotations on screenshots can also be helpful to explain problems/goals. Please use the "secure" tab for confidential information