Greetings, fortunately I found out the reason accidently
Today, the rear arm (Qenchi) broke from where it was attached to the chassis. I replaced it with one from old parts market, and after that the car is fine n OK.
Conclusion: this arm was breaking which was caused the shuddering. Bcuz it is a RWD car, thats why it had that problem. I ran this car upto 110Km/h (provided its on CNG)and no shuddering. I guess I am squeezing more out of a 1978 model. Anyways, thanks all guys who have helped me. May Allah bless you all.
My mechanic told me to replace the rear coil springs as well, bcuz they were not changed when i installed the CNG kit. He says that these springs are weak and while car was loaded it broke the arm somewhere on the road.
Anyways, I m happy now.