Yep. Exact same thing happening to me. Although, it seems not only in crowded places.
My connection is fine - I'm on a gigabit connection with AC wireless. Only WoW is messed up.
Random ultra lag spikes that can last 10-20 seconds and then catch up. Or, like you, I just stay lagged until an eventual disconnect a number of minutes later.
I've reinstalled the game, disabled all addons, tried running it in different compatibility modes ... and nothing works.
It's becoming unplayable. I haven't been able to find a solution anywhere .....