Thumbnail Z index problem

I recently set up my site with woocommerce and now
when/if the browser is small (vertical wise) and I scroll down the page
the woocommerce content sits above my main nav menu.

the site: The Precise Life

Seems like a z-index problem and ive tried using firebug but still
cant pinpoint it.

Heres an image to show what I mean
https://postimg.org/image/4zkevcjvp/