IE7 is driving me batty. How do I fix this? CAN I fix this? (testcase inside)

For your perusal: http://roothorick.mooo.com/ie7-more-zindex-madness.html

Open it in Firefox, or Chrome, or Safari or IE8 or IE9 (probably IE10 too) and hover over Two or Four. The absolutely positioned sub-menus are on top of the other elements, like they should be.

Now try it in IE7. Some really bizarre behavior ensues. Two's subnav is above One and Two but BELOW Three Four and Five.

I just can't seem to wrap my head around why this is happening. Even if each element was given its own stacking context, why would some be below the absolutely-positioned ULs and others above?