Menu moves when browser is resized
So I'm taking over a couple websites for a student job at my college and I'm trying to fix some of the problems with the site. I plan on redoing this menu completely in the future but for now I just want to fix it. So one of the sites has a problem with the menu moving left and right when the browser is resized,it resets if you refresh the page but I'm at a loss on how to fix it. I'm sure it probably has to do with the css and positioning but so far I've been unable to figure it out.
The site is:
heres a link to the css page that controls the menus. It wouldn't let me post the code for some reason.
Any advice or pointers would be greatly appreciated!