Posts Page margin-top changes on site reload

[sir-brickalot]

Hi, on a clients site I have a strange behaviour on the Posts Page:

When navigating to the Posts Page via menu entry, the last post is shown fully right under my main menu.
When reloading the page, the title of the most recent post is cut off and hidden behind the main menu.

There is in inline element that controls the margin-top:
<div id="page" class="page-container" style="margin-top: 199px;">

This element changes when reloading the page to:
<div id="page" class="page-container" style="margin-top: 86px;">

Right now I have this mitigated by adding:
@media only screen and (min-width:768px) { .page-container { margin-top: 199px !important; } }

Maybe you can replicate this problem? If this is an error on my clients end, than I'm sorry to bother.