I played around with the circuit a little and it seems that the problem is with the circuit only.
My reason for saying this is because as soon as I connected the ground of my oscilloscope with the 12V ground which comes from an external source (smps), the flickering suddenly stopped and glowing of the bulb at different brightness levels was just as expected, smooth and perfect. As soon as I removed the ground of my oscilloscope the flickering started again.