I have a similar problem when i join in on high graphic content activities. I joined a Naxx raid on Saturday and Disconnected on my main 4 times (my graphics were set to super high) and disconnected on two of my slaves (graphics set to med/high). I turned down the graphic settings one notch .. restarted and didnt disconnect. You may need to go into the video settings and adjust the "quality" down.

I also have (and use) GIGE connections and have no issues with DC (except for the activity noted above).

Could be a possibility and worth checking out.

Sojólly

Machine 1 - Main - Q9450 - 8GB RAM - GTX260 - 24" Display - Gigabit Ethernet - Windows Vista 64 Ultimate - KavoomKM! - Keyclone - G15 Keyboard - Microsoft Wireless Notebook Mouse 4000
Machine 2 - Slaves *4 - Q6600 - 8GB RAM - GTX 260 - Dual 20" displays (Rotated sideways) - Gigabit Ethernet - Windows Vista 64 Ultimate - KavoomKM! - Keyclone - No keyboard - No Mouse
Network Switch - Gigabit Ethernet (this allows for superfast traffic between the two machines)
Router - Linksys WRT54G (wireless is for other computers)
Cable Modem --> Comcast