Try this:
http://pcpitstop.com/
Run the tests and see what they say. Also contact your internet provider, Are you DSL or Cable? Have them ping your modem and run a line check. If PC Pitstop has no recommendations and your ISP says your clean then try sending in a trouble ticket to Blizzard. Note just found this see if it applies to you
http://wow.incgamers.com/forums/arch.../t-375771.html
Good Luck