Virtual Machine loses netcard connection

Hello.

I have installed 2 virtual machines on virtual Server 2005 R2.

The problem is that the netcard of the virtual machines (Windows XP Pro) from time to time loses the connection to the network. It reports that the cable is unplugged.

In order to solve the problem I disable/enable the netcard.

Any ideas what might causing the problem?

Should I change physical network adapter?

Thank you in advance.