On my site http://www.theportwebdesign.com/ – on Chrome – dropdown jump and we cannot catch it. It drops down but when you put the mouse on it it disappears.
Once I scroll down the menu scroll down with it (mystickymenu) then the dropdown works. I use mystickymenu on almost all my sites so this is not that that creates the problem.
We found that your site is working good at our computers. If you are still facing this problem please explain briefly about the problem and screenshot might be helpful.