I'm experiencing a similar problem.
I'm on OS X 10.5.6 Host w/vbox 2.1.0. I'm trying to network an Ubuntu Server 8.10 VM with my LAN so I can access the VM IP address for website testing (port 80). I enabled host interface sharing (Airport is the host interface) and setup a static IP inside the VM. It seems to work for 5 minutes, then my net-connection slows/stops (on host and VM) then bounces back to normal. It repeats this slow/stopping every 5 minutes, but appears to work fine during those 5 minute intervals. Anyone know why this is happening? Thanks for any help.
Update: I may have solved the problem with some help from erstazi in freenode.net/#vbox. As noted in the User Manual in section 6.5, Currently, VirtualBox supports only IPv4 over AirPort. With this in mind, I disabled IPv6 on my Airport network settings.
System Preferences > Network Pane > Airport > Advanced > TCP/IP > Configure IPv6: Off
Update again: Spoke too soon. With IPv6 disabled, my connection lasted for about 20 minutes initially, but again, it cuts off on the host for a minute or two, then is fine for another 10-20 minutes then cuts off again. I don't get it.