window_open using _top only works after clicking twice

I can't properly switch from frames-page to non-frame page.

On http://www.panglas.ch/glasbau.htm click on a picture on the left hand-side.
Then rollover "Über uns" and click on "Lageplan". This gives a not very useful
page though the _top Option is being used. Click once more the page looks as
expected.

mm_menu_0516235514_1.addMenuItem("Lageplan","windo w.open('lageplan.htm', '_top');");

Any help is appreciated
Brigitte