Sometimes my cards do the same, I run thread concurrency 8192 and the intensity of 17 on second card and 13 on the main card.

Usually a few restarts fixes it, I find when its got the low hash the GPU usage is 100% where as when they are working correctly its 98-99% usage.

I would look into drivers first of all, strip off all drivers and install fresh. Also I find stopping the miner and changing the clock speed slightly using the overclock tool and starting miner again fixes it.