Baffling IE6 Display Issue

I've almost finished fixing this site for compatibility with IE6:

http://catalysto.com/imsta/

However, as you'll see, part of the background image of my Nice Menu is obscured by white blocks. I've spent around three hours tweaking every single css component and haven't gotten anywhere.

It seems that a white band across the top is being displayed, except for on the "#navbar ul" part which remains blue as it should.

Any suggestions? This job is desperately urgent; I know my client is using FF but I've a sinking feeling that he's going to show it to someone with IE6 before I can fix this!