WoW:CVar timingMethod

Blizzard says[1]:

There were some improvements made to better support Dual-Core CPUs.

This will require adding the following CVAR to the Config.wtf file located within the WTF folder inside the World of Warcraft install folder:

set timingMethod "1"

You can also type this command in-game, but will require a restart to save the changes:

/console timingmethod 1

If this does not help try setting the value to 0.

Blizzard says[2]:

timingmethod 1 is generally for systems where the cores do not synchronize.

You might run into issues like your framerate caps at 64 instead of your vsync value.

timingmethod 2 is a higher resolution timing method and you won't hit the cap with that.