Menu Items Move When Window Is Made Smaller

CSS: https://pastebin.com/4fnHJji7

Hey all,

Im having trouble figuring a problem out. The site is https://www.steriletechllc.com

I noticed that if the window of the browser is dragged in to where the menu is, the menu items will begin to stack vertically as the page is made skinnier and skinnier. This must be a CSS issue but I cannot figure it out.

Thanks in advance!