Try this

setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_USE_SYNC_OBJECTS 1
cgminer --scrypt -o STRATUM -u WORKER -p PASS --thread-concurrency 24500 -I 18 -g 1 -w 512 --gpu-fan 70 --lookup-gap 2 --gpu-engine 947 --gpu-memclock 1250 --gpu-powertune 20