parallax / css conflict in Chrome for custom theme

Hi guys

I'm working on a custom theme and I'm seeing an issue in Chrome where some elements are disappearing once you scroll down to a parallax section of the page.

You can see that a grey sticky nav bar appears as you scroll down the page and it all looks (although very half-finished) ok.. Please see what I mean here:

Keep scrolling and once you get to a Parallax section with a bunch of Lorem Ipsum, most of the items in the grey sticky nav disappear!

I've played for hours with z-index, position, overflow and other properties but cannot get this to play nicely. I've also tested using Firefox and Safari and this issue does not happen - it's only in chrome.

Any help would be massively appreciated as I'm getting hung up on this rather than getting on with the mountainous task of finishing the theme!