Website Suddenly Unresponsive

Good Afternoon, folks: I am in the process of re-assembling a website I have migrated from Joomla to WordPress. It is almost finished, but since about a week ago, it is no longer responsive on cellphones and my tablet. The responsiveness feature has always worked fine, as tested in my Kindle’s Silk browser and a colleague’s Android phone.
Responsiveness survived my recent update to WordPress 3.8.1, as well.
I have not made any significant changes to the site since it was responsive, except for disabling some little used plugins, and reassembling a few more posts.
Today I tested it here: http://www.mobilephoneemulator.com/ in some emulated browsers. It was not responsive in any of these simulations.
WHAT I HAVE DONE TO TROUBLESHOOT:
I ran through Zed’s troubleshooter as a guide http://www.cryoutcreations.eu/forums/t/minimal-step-by-step-tutorial-for-debugging-bugsissuesincompatibilities
And took the following steps:
Made sure responsiveness is enabled (it is)
Updated to the latest version of Parabola v1.3.3
Updated all plugins
Disabled all plugins (no change)
Reenabled plugins
Switched to 2014 theme (still unresponsive)&
Tested again at : http://www.mobilephoneemulator.com/
Switched back to Parabola
I don’t think I have any caching plugins to deal with.
I tried a debugging plugin which errors indicated that I need to update php from 5.2-5.3, and that one of my plugins has an undefined variable for the install date. But these errors were likely present when responsiveness was working.

None of this made any difference, except that disabling the plugins and switching themes broke the website. The majority of my theme customizations are focused on replacing the topmenus with side menus, and adding some sliders for photos in order to replicate the look of the original Joomla website http://www.seahec.org

So, having exhausted those steps, what might be the next steps? I would appreciate any guidance people could offer.

Thank you.