Upfront Theme not loading consistently

Okay. So I have the site about as streamlined as I can make it. Plugins are limited to a handful and all but one of them are WPMUDEV. But the site is so inconsistent with loading all of the images! Sometimes it's fine, other times the background color shows but not the header or background image. Sometimes the homepage loads fine and then the interior pages refuse to load the header.

My Internet connection isn't great today. Is that the problem? How do we overcome this? If a client overseas doesn't have the best connection I still need him/her to see the site. Is there a diagnostic that can help with this?

I ran webpagetest.org and last night it was 1.76 seconds but today it's 4.18. I definitely see a bigger issue in IE than in Chrome or Firefox.

Help!