About menu CSS

Hi,

I've configed the menu of this site http://www.nhadep.net/vi (using CSS, i copied menu's code from style.css to custom.css)

But sometimes when the page loading, the menu were loaded the old CSS (please take a look the screenshot). I'm not sure this is a bug or the file custom.css being replaced by style.css before the whole page finish load.

How can i fix this issue? Please advise.

Thanks,
Tuan.