When is this happening?
Do you have addons installed? (If so, have you tried without the addons by temporarily moving your Interface, WTF and Cache folder to another place, ergo creating new versions of said folders when you re-try starting the game)
Does it seem at all to have a pattern to when it happens?
Does it happen in any other game or program?
Either of these can help us figure out what might be the problem!
I’m myself experiencing disconnects when I log in to my main because of addons, and I have been trying to figure out a way to fix it for a while now, using only a work-around where I disable all addons → enable once in game.
Happens purely on my main, no other toon.