I'm not sure a permanent fix is going to be possible. It looks like the 'fitvids' JavaScript library might not have been designed with multi-column displays in mind.
However, here's another suggestion which you might like to try.
(1) Set the 'Body text layout' to '1-column wide'.
(2) Add your video iframe embedding code to the editor, such as:
<p class="vid"><iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/CH880_VrxxU" width="560"></iframe></p>
<p class="vid"><iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/CH880_VrxxU" width="560"></iframe></p>
<p class="vid"><iframe allowfullscreen="" frameborder="0" height="315" src="https://www.youtube.com/embed/CH880_VrxxU" width="560"></iframe></p>
(3) Add the following to your 'showkase/_themes/kosel/css/custom.css' file:
@media only screen and (min-width: 768px) {
.body-content .vid {
width: 49%;
}
.vid:nth-child(odd) {
float: left;
}
.vid:nth-child(even) {
float: right;
}
}
The suggestion above, using a single-column layout for your video page or disabling 'fitvids.js' are possibly your best options. (I would probably edge towards a single-column layout for your video page as this will retain the responsive video functionality that 'fitvids' provides.)