Menus retracting

I have inherited a site.. http://www.life-church.co.uk WP 3.0.4. Theme Barista 1.0.4 and have been administrating it for a time. Have just started experiencing a problem with dropdown menus with some computers. When you hover over header – menu drops, as you move the curser down – the menu retracts therefore unable to select page! If you move the curser down very fast the menu stays and you are able to make your selection. Is there an easy solution? I am limited in experience of coding etc.