Not sure what your settings are or the cards or OS but sometimes you have to increase the power to the card that isn't as good as the others. This happens because the miner is pushing the card more it sometimes can't handle the extra demand. In windows using afterburner, increase the mV to the core. I have once rig that has six of the identical cards but there is one card that can't quite handle the under volt like the rest. So, I turn the mV up a bit for all of them and its runs rock solid. Its worth a try at least until we have more information on your setup.