Menu Jumping Up and Down

pretty straightforward answer to this question.

it appears to be a feature of the Twenty-Fourteen theme that you are using – when a header image is used. this earlier thread also suggests a potential cache issue?

http://wordpress.org/support/topic/problem-with-tophorizontal-menu-bar-with-header-image?replies=16

I been able to recreate here with a clean install, using a header image – and the menu pops up and down.

the jumping menu functionality is created within functions.js of the theme – if you want to dabble in there.