unstable ram

Hi, i had two 2GB DDR2 800 MHz Kingston CL 6 RAM and had 6-6-6-18 timings. I've set timing 5-5-5-18 and played CS:GO for 30 minutes system frozen. I've tried to set it stock timing (6-6-6-18) but can't get display and i've reset the CMOS. Now I've set 5-5-5-15. But i can't understand why i can't...