I am having this problem also, except changing auto widths to fixed widths did not help me. I am still having trouble with the sub menus going away on IE 8 (compatibility mode) and I assume that's for IE 6 and older. I created a page to work with this containing only the menu to try and isolate the problem.
Not really sure what to do next. Help!
By the way, only the services section of my menu has flyouts.... I would really prefer to use auto widths for the main and sub-level but if I have to use fixed width I can live with that. It's just that it hasn't helped up to this point. It works fine everywhere else but for older versions of IE and IE 8 compatibility mode, the flyouts go away as soon as you try and drag over them.
It seems ridiculous to be supporting such old software, but as long as my client wants to stand behind his 25% or so users still on older versions of IE (proven by the site stats), I gotta make it work somehow.