Same for me, everything between 11.11 and 12.4 does not work. Now I am minning w 5770 all the time with intensity 11 and it does not slow down my computer. Try to run cgminer with this command, works best for me:

cgminer --scrypt -o stratum+tcp://middlecoin.com:3333 -u YOUR_BITCOIN_ADRESS_HERE -p x --shaders 800 --intensity 11 --worksize 256 -g 1 --auto-fan --gpu-engine 750-900 --temp-target 79 --gpu-memclock 900

if it crash, try remove " --auto-fan --gpu-engine 750-900 --temp-target 79 --gpu-memclock 900"