I don't think so. A bottle neck doesn't mean fps fluctuations. Bottlenecks are stagnant , meaning your frames will just hit a wall. My first guess, is your Drivers or maybe the third GPU is faulty?
Either way, to figure it out, it's no use looking at the GPU usage... at 1080p you are never going to stress a 670 tri-sli setup. Another thing to check is your PSU, sometimes too much up and downs is heat/power related, aside from drivers or games just not being good with the sli setup.