When hovering over link, website randomly changes the hover background size

When hovering over link, website randomly changes the hover background size

While working on my website, I have decided to incorporate PHP just to
play around with. I figure I'm paying for it in hosting (sort of), so
might as well use it. The problem I am having is when I hover over a link
in the nav bar I have made, the hover "decoration" that I am using changes
the hover background height. Problem is, it does it on every page BUT not
all the time. Seems random. For some reason it seems to be doing it more
on my about page but I have seen it do it on others. I haven't made any
changes to the others to make them stop nor any changes to the about to
make it start more.
The site in question http://www.journeytomyoasis.com
Since I can't post images yet, here is a link to a screenshot
http://journeytomyoasis.com/images/common/hover_error.png
Code that is in the top of about page
<?php $page_title = "About | Journey to my Oasis"; ?>
<div id="fb-root"></div>
<script>(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/en_US/all.js#xfbml=1&appId=15681725508";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>
<?php include("includes/header.php");?>
<?php include("includes/navigation.php");?>
code in the navigation.php
<!-- Begin Navigation -->
<nav>
<div id="nav_container">
<div id="menu">
<a href="http://www.journeytomyoasis.com/about.php">ABOUT</a>
<a href="http://www.journeytomyoasis.com/blog">BLOG</a>
<a href="http://www.journeytomyoasis.com/portfolio.php">PORTFOLIO</a>
<a href="http://www.journeytomyoasis.com/blog/journey/">JOURNEY</a>
<a href="http://www.journeytomyoasis.com">HOME</a>
</div>
</div>
At the top of the about page I included the Facebook code that I am using
so that you all know it is there in case you all might think that is it. I
have poured over the code a lot but I cant see an error. The fact that it
is the same php file used on all pages and every page has had issue at
least once but not always confuses me.
I have tested on Firefox and IE 10
Thank you all.