I had this problem: When I clicked on a horizontal menu item, the submenu items expanded beautifully. But as soon as I moved the mouse to pick a submenu item, the submenu items dissappeared! This was happening in IE7.

Solved by editing the file \modules\mod_ariyuimenu\mod_ariyuimenu\js\assets\menu\sam\safe_menu.css and commenting out three line-height statements (the ones with 1.45, 1.5 and 1.9 value).

I hope it helps