Thanks for posting these data.  I noticed that you have a "clear:both" property set for your <li> tags in the menu.  Why did you choose to do that?  I suspect that that is what forced you to comment out the other rule in IEMenu.css.  Have you tried removing that "clear" property from that rule and uncommenting the rule in IEMenu.css?

Generally, if I were you, I would compare your rules against those in the sample page (for Menu) provided in the kit.  Where you see differences in things other than font, color and other pixel rendering properties (i.e., where you see things that affect positioning dramatically like "clear") I'd be suspicious.

Hope this helps.  Keep up the good work!