Networks

I have a small network consisting of 1-Dell 2900 16GB Server 2008 x64, Exchange 2007, 1 Domain.

We have about 35 Windows XP workstations. At seemingly random times (could be twice a day, could be twice a week) users appear to be disconnected from the Domain (but not the network in general). However, the server can be pinged. RDP still works to it and to workstations. Exchange still works. Internet still works. Only file shares are inaccessible and all users will have an offline files icon in the lower right (my documents are synced to server). This has happenned since the first week of Feb when we installed the simple network of server & 12 workstations. In an attempt to fix these disconnects I have: disabled the onboard Broadcomm 5708C NICS, removed the TOE module & disabled receive side scaling; I Installed teamed Intel PT 1000's. Added a BDC. Replaced all switches (Dlink DGS-3100 layer 2. None of this has helped with the disconnects. Only rebooting the PDC (The same server at the top) fixes this problem. I can induce and replicate the problem by taking additional workstations and joining them to the domain. After about 3 or 4 (sometimes just 1), users will start getting 'offline files' and not be able to access shares.

Help?