Look at this for a laugh, dropped my res to 1920x1080 with crossfire still enabled and look how low my framerate still managed to dip.
http://cloud.steampowered.com/ugc/578931676592782717/C993A7B4541FB9C6EA0E2223B7E769939B1E178C/
And that wasn't the lowest, i seen it spurt down to the 60fps range. Top marks to treyarchs coders, they sure know how to manage to totally balls up a game.
Matter of interest G, I am totally impressed with 11.8s on Black Ops (I reported this in thread in Gfx forum) as they seem to have nailed fps drops for me, have you chanced just using a single GPU? (or 11.8s and latest CAP perhaps with ur cf?)
..... Maybe your problems are caused by camping with a crossbow?