Cadfael at Elitist Jerks has written a guide to fixing disconnecting in raids. It's long and detailed, with several fixes that can help if you are suffering from:
- Sometimes disconnects after porting / logging in to Dalaran
- Disconnects at heavy raid AOE damage and events like Thaddius, Gluth
- Chain disconnects in general
- Excessive disconnects at Sartharion
- Excessive long phase of "red question marks" after a patch when the client's cache was deleted or you don't see NPCs and player for a long time or even disconnect seemingly unmotivated after a new patch often
- Often disconnect after porting in or out of an instance
It's an extremely good article that has a lot of suggestions that can easily be implemented.