IE7 behaving badly

I'm seeing IE7 behave really oddly depending on how wide the window actually is.

I'm viewing a test page at three different widths, and the page reflows dramatically differently depending on the width i use. The w3c seems to think that the page is formatted properly.

Here are the three, in order of increasing width (click for full-res versions):

1. Original width:
   
2. A bit wider:
   
3. A bit wider still:
   

Sigh. Why on earth would the middle frame cause the main body to drop down like that? Is there something we can do about the page?

Since i'm currently without a Windows box, i'm running these tests with IE7 under Linux. Maybe the kludges involved there are at fault? Does someone with a Windows install want to confirm or deny this craziness?

IE6 doesn't appear to have this problem.