Problem with ports while using NAT

[DenFm]

Hi,

I have Windows Server 2008 x64 and latest version of VirutalBox
On the virtualbox i have installed ubuntu server x32 12.10

In it i launch 4 counter-strike 1.6 gaming servers on different ports 27015-27021.
Settings are via NAT. Ports are forwarded.
Everything launches great, people enter servers and play, everything goes fine. But some time later cs-servers are not being seen because of NAT.
Ports are not working until I restart ubuntu on Virtualbox.
They can be working for 2-3 hours, or for 20 hrs in some cases, but still some time later they stop working.

What can be the reason for all this?