gpu mining with sapphire 7950 dual x shutdown problem

i am facing shutdown problem sapphire 7950 dual x
i am using this script in cgminer
cgminer --scrypt -o stratum+tcp://pool.com:3322 -u user.worker -p x --intensity 13
it work fine but giving me just 250 kh/s to get high kh/s i have to change intensity 13 above but when i try 14,15,16,17,18,19 my computer direct shutdown and screen blank..

please
Your cards cannot support that intensity, probably overheat. I had that problem too last time when I set 18~20 intensity. help me..