Submenu displays while page loads

Hi there

I am quite new to wordpress, and I am relatively inexperienced with web design/development. Now I have run into a bit of a strange problem.

The website is available at:
http://microvetonline.co.za/wordpress/

The problem:
Notice that the Microvet submenu displays while the page is loading. I have checked the style sheet, the submenu’s CSS does contain the following entry:
Display: none;
I noticed this problem for the first time when I fiddled with the parent menu’s layout.
When I changed:
left: 10px;
to:
float: right;
The idea was to move the menu to the right hand side of the screen.

Then this problem cropped up. Does anyone have any ideas how to fix this?

Any input/advice will be appreciated.