Users are being disconnected from Network Resources

I have about 20 users in our local office all running XPsp2 connecting to a Win2k3 AD. It seems that days that all users are in the office, a few users keep getting dropped offline. They can be lookin inside a Network Share and hit F5 to refresh and then tthey are told that they are offline. Hit f5 again and they are back online. It seems that it is worse if everyone is in the office. When not a lot of people are in it is not a problem. I have checked the licensing and it all ok.