Fonts jiggle after menu goes back to top (CASPER)

I decided to redo my site using casper for all the best features and support. I have now started modifying casper and changed the top nav weight. However now when i scroll down, and then back up to the top of the menu the font jiggles a bit. Any idea why?

Here is a video of exactly what I mean:

My site:
healthzam.com

Thanks,