Fix header/menu "glitch" when user scrolls down page (seems to be jQuery issue)

Hi. I am getting a weird glitch on certain pages. The glitch occurs when a user scrolls down the page. Specifically, my main menu in the header will "bounce" when it scrolls past certain jQuery elements on the page and sometimes the z-index on this menu will get messed up (the jQuery elements appear on top of the menu momentarily) when I scroll up and down fast. These glitches seem to only occur when scrolling past the slider or testimonial section ([login to view URL]) or gravity forms. I want to fix this odd "bump" or "bounce" that can be seen when scrolling and the odd z-index issue. The odd thing is that these issues are intermittent and barely noticeable. They sometimes don't occur (usually the glitches can be replicated when the cache gets cleared). I am getting no errors in my console log. Someone please troubleshoot this and help me. This is a custom built theme so we have complete control over all libraries and files. You can see the glitch on both of these websites: [login to view URL] and [login to view URL]