how did you test for the bad core? my hash rate goes from 800 ghps to 14 ghps. Very unstable keeps restarting because it runs at less than 100 ghps for 5 min.
BFGMiner allows you to look at each core (press M and then use cursor to cycle through the list, a bad core will either be in RST state and/or show 0 ghps)
If you look at hashrate, always look at what your pool tells you and let it run for at least one hour before you take the values for granted. Local hash rate in current cgminer is all over the place really.