Darkhaze,
With the game closed please try moving or deleting your WTF, CACHE, and INTERFACE folders out of your World of Warcraft directory. These are temporary cache files that will be recreated by the game the next time you launch. Moving them to the desktop makes it easy to replace them if necessary.
Please also note these folders contain your saved macros and UI addons. By moving or deleting these folders you will also remove any saved preferences, UI addons, or macros you may have added. However, in order for us to troubleshoot the issue we need to remove any modifications to the game and make sure we are diagnosing the default settings. If you're using Windows Vista, please remove those folders from the following location as well:
C:\Users\<your windows account name>\AppData\Local\VirtualStore\Program Files\World of Warcraft
Once you've done that, please try again and see if you continue to have the same issue.
Make sure you're using the latest network card drivers, then check some network card settings. The checksum offload setting, duplex setting and a router's firmware.
Please first review the instructions here for the checksum offload and duplex settings:
http://www.blizzard.com/support/wow/?id=aww01686p3
Do you have a router within the network? If you do, try temporarily bypassing the router by connecting directly to your modem with an Ethernet or USB cable. This may help isolate the cause. If that seems to resolve the issue, contact the router manufacturer for assistance with updating the firmware.