I'm having the exact same issue on one core, it's throwing HW errors... the other 3 cards are fine.
Config;
setx GPU_MAX_ALLOC_PERCENT 100
color 02
cgminer.exe --scrypt -o stratum+tcp://fr.dgc.theblocksfactory.com:3338 -u abc -p x -I 13 -w 256 --thread-concurrency 8192 -g 2 --gpu-fan 100
I've set the voltage on the cards to 1150, even at stock, it throws HW error.