try with
set GPU_MAX_ALLOC_PERCENT 100
set GPU_USE_SYNC_OBJECTS 1
sgminer.exe -k quarkcoin -o stratum+tcp:// -O -p d=0.5 --intensity 18 --worksize 64 -g 2
leave -p d=0.5
with core clock 1500
and mem clock 1000
i'm getting now 11MH per card
if you still have problem restart the PC and try again