virtualbox.org

Inside my virtual machine, I often run scripts that cause a flurry of network activity (namely, lots of short connections to different servers / websites, on a variety of ports). Frequently, I will get 80% through this and then the script simply hangs, refusing to go further. A quick test from the guest reveals that the website that the script is attempting to access is unresponsive, even though it works fine from the host.

I am using NAT based networking. Could it be that I am filling up some NAT table or something?

There is a fix similar to this mentioned in the v4.2.2 changelog. You might want to give it a try.