Twentyten Childtheme footer.php breaks navigation dropdowns

As a beginner I’d appreciate some help building a child theme.
I seem to have got along OK changing the stylesheet, now I want to change the footer (I’ve looked at the footer widgets and they don’t do what I’m wanting).
The problem I am having is that if I copy even the original footer.php into my child theme it seems to break the navigation drop downs.
A small gap appears between the titles on the navigation bar and the drop down menus.
Worse, this is not just a cosmetic issue.
As soon as the mouse is moved down to select an item from the dropdown the dropdown menu disappears so the item cannot be selected. Speed does seem to help but that is not good enough.
Any idea how to overcome this?
Many thanks for any help