Top Nav items stacked instead of Horizontal

Site URL: http://www.breatheportugal.com

On my website (www.breatheportugal.com), I see an unusual configuration of the nav items on the top.

Sometimes they are displayed horizontally (this is the right way), but lately, sometimes, they are stacked on top of each other.

I attached two images with the situations described above.

I know it does not necessarily has to do with the responsive design because I am seeing the website on a 13-inch full screen and it still shows them stacked.

Thanks in advance!

Ajax loading issue, could be caused by slow internet anywhere along the line. I have had some strange thinks happen too, lately.