Guest is losing internet connection (NAT)

Hello.

I set up a virtual machine using workstation 9 and since then, guests (windows xp pro sp3) are losing internet connection often, stopping programs and downloads. It happens, more or less, every 3 or 4 minutes. I do have internet connection most of the time though. When it happens, in the bottom right corner, a message pops up "network cable unplugged"

I have workstation for both windows 7 and ubuntu 12.04

The issue I'm having only happens in ubuntu; If I load the same virtual machine in windows I won't have any problems.

I'm using NAT and DHCP.

Any ideas?

Thank you!