Yep.. says coming soon AND I think your security cert may be off as well.. getting security warning.
Something to look at that "may" be why it is happening sporadically is your wanting https, as I stated above you do have a cert problem but also, i think you are calling the non secure version of js for the mailchimp form.
Although you made that CSS change for the background color in your "local" files.. I'm wondering if mailchimp is calling it's own hosted files (CSS), thus giving you the default form look / CSS.
Personally, I would fix that cert problem and then be SURE that all files that your site calls on from out side your own site / server.. is for https