This sounds like IE's z-index bug. There should be a wealth of information about it that you can find via google, but more helpfully, the issue has been discussed on this list with regard to Superfish quite a few times so if you search this list specifically you should be able to find the solutions you need. I've attempted to describe the problem and the solution on this list myself, so have a go at finding those specific threads to start with.
Joel Birch.
