I sometimes experience huge FPS drops - about from 150+ FPS to 15-20 (playing WotLK, Ragnaros)
This happens ONLY in PvP, both in BGs and World PVP (city raids etc).
It happens very rarely - a few times a month max - but it's ALWAYS when I'm against good / geared people, typically from the same few guilds, which led me to believe it's some sort of exploit.
For the record, I've got a good rig (4690K, R9 290), no problems with any game and good, stable temps - so I would rule out any hardware issue.
^ Exactly that.
Don't play with addon versions that weren't made for wotlk.
I don't have any non-wotlk addon.
My addons are: Adibags, Atlas, Auctioneer, Necrosis, Quartz, DBM, Turing Test anti AFK (it's idle outside bgs) and AH-fix
So I don't really have many addons unlike some people.
Besides, I just spoke about this with a friend and he also experienced the same issue, even with all addons disabled.
I remember, when I got a lag spike during a SW Raid, that other people started complaining about that very same issue as it seems to affect everyone in the area.
I can't really disable addons and test it because it happens too rarely.