Well I logged in at the bank and was walking round outside and lagging <30 fps) a bit. Worth noting that I throttle some windows to 20FPS background and otehrs to 10 FPS background. ) Strangely I can ususally fly over stormwind (even past the bank etc) np. I only lag when I get on the ground with all those people jumping and spinning. And its a constant thing, it doesnt go away. I have experienced things in the world very occasionally that might be closer to what you are thinking of, where I lag for a few seconds then im fine again. This also might be related to phasing, as it happens about 200x more in phased zones.

As I posted above, video card is a radoen hd 5870 eyefinity 6 card (2 gb ram), 16 gigs of ddr3 1333 ram, and 890fx chipset.

Wow is on the windows drive (C) which is also my SSD.