Valve was removed by the mech, in May 2011, because of the problem we are discussing.
Dear Skill, thank you very much for the expert opinion :P, points taken well. I will do as directed and inspect all of the cooling system.
I have understood the function of thermostat valve and will put it back in today. The car taking two mins before leaving vibration and picking up, that was because of the missing valve.
I have never seen a decrease in the coolant level in the radiator, changed coolant a month back and washed the radiator by flowing fresh water in it. There is no leakage.