VirtualBox-4.1.16: network is unstable in guests

[Yuri]

I notice that probably after some recent vbox update network became 
unstable in guests.
I observe the periods when all traffic in guests drops to zero, torrents 
stall, browser can't connect, ping says host unknown.
And later network comes back. I see the same in Ubuntu64 and FreeBSD 9.0 
amd64 guests.
Restarting network in the guest doesn't help.

Nothing like this happens on the host, network is stable and never goes 
away.
Host is 9.0 amd64.

Yuri