Browser gets confused, partially rendered pages

I'm not sure if this is a beginner's issue, advanced, browser-specific, or specifically related to Upfront / Parrot. Please move to another forum as appropriate.

On page refreshes I randomly get pages that look like the attached image. Partially rendered, no content, just the raw theme. The URL is correct. This can come from the home page, wp-admin, or any other page. F5 to refresh sometimes helps. If not then ctrl-shift-F5 or ctrl-click on the browser refresh icon. Sometimes I need to do a fresh like this several times. This is Firefox v43.0.x.

I'm just starting to style this site which might explain the default Parrot data shown here. I'm not updating styles while I'm doing page refreshes - it happens randomly while I'm doing other things.

I just activated WordFence which does caching, but this happened before that. There is no other optimisation yet, no CDN or other outside interference. I'm wondering if the server or browser are choking on the number of scripts or some network delay. I'm going to start checking logs but wanted to get this posted first.

Any ideas so far? Has anyone else seen this?

Sure, this is a new site in development but if this turns out to be an issue with Upfront or Parrot then I need to find out soon so I can work out alternatives.