Oh it is definitely the MailChimp code. I’m not even using their widget. I just copied the source code of the opt-in design, stripped out the mchimp garbage code, sized it to fit the sidebar. But, when I remove the code . . . the theme bounces back to perfection.
Nope, didn’t make any core theme changes. And, thinking it may have been my careless stripping of code I went back and tried it with the whole code and it still breaks the page. It’s mailchimp for sure.