Oh! the master niresh himself. Thanks for responding. I had done it earlier but it did not work that time. But at this time with confidence because of your response, i did it again. It works not 100% but yes, works. sometimes i have to boot with nv_disable=1 and then re install nvidia driver again(though on my 660 web drivers are not enabled but default osx drivers are), but still re installing does things right. Now the frequency has reduced to 1 out of 10 restarts. which was earlier 8 out of 10. maybe bcoz i have touched some other system files too. so now re installing sierra again fresh. hope now it works 100%.
Thanks for your kind response. god bless you.