Shared networking problem with multiple guest OSes and 5160

I didn't try this prior to build 5160, so I don't know if it was a problem there.

I have two guest OSes. One is XP, the other is Ubuntu 7.04. I have both set up for shared networking, where the Mac acts as a NAT. I prefer to do this so that I can switch between wired and wireless networks at will without having to futz with the guest OSes, I can easily access the Unix OS (scp/ssh/etc.) when I don't have a network connected, and when I'm roaming and have to buy wireless somewhere, I have full access to the internet from my guest OSes.

I get an IP for my Linux OS as 10.211.55.6, and my Windows OS as 10.211.55.7, so no IP conflicts.

here is the problem. One of the OSes "loses" connection to the internet. Its IP is still there, but if I generate any requests - through the browser, ping, whatever, one of the OSes doesn't get any response (time outs, etc.). What I've noticed is that if I reboot the guest OSes, whichever OS comes up "last" ends up keeping its connection, while the earlier OS loses its connection. So, if I'm operating just fine in Ubuntu, then boot my XP OS, the XP will be able to access the internet, but Ubuntu loses the ability to do so.

For now, I'm working around the problem by having my Windows OS get its own IP on the host, and having the Linux use shared, but this is very annoying.

Any thoughts from gurus out there? My initial thought is that there is something broken with NAT - it was only expecting there to be one guest OS running, for example, and is always sending responses to that OS as opposed to the OS that made the request - as if it maybe only has one entry for translation and is using the last OS to boot as its translation target.