Fixed Nav Display errors

It seems that on mobile, after the shiftnav hamburger is clicked, the fixed navigation header of the page is re-rendering as the page is scrolled. Meaning that the fixed header gets all twitchy as the user scrolls down the page. It keeps trying to scroll the fixed elements even though they are position fixed, this results in the navigation flickering and pulling away from the top only to snap back in it’s fixed position.

I’m working on a password protected site here, so I can’t supply the link, but I did find a way to fix the problem – which then caused other problems. The base overflow from the first line of the shiftnav.css is set to ‘hidden’, if I change that to ‘initial’ the flickering header works as expected and stays put. The problem then is that the shiftnav menu can’t be scrolled vertically after that, so it becomes unusable in landscape view.

I’d really like to get this resolved. Have you heard of others having similar problems? Anything you might suggest i try? If we haveto I can strip out elements and get a test site setup to work this out.

Thank you very much.