Here we go for updates....
As explained to me initially MASS technician changes tank and the pump together. After that when they tested the vehicle they found sluggish acceleration at certain intervals. But while second TD they found no problem so they washed the car and got it ready to deliver me.
But after washing the car it refused to start again. Then after testing the wiring which was initially found ok was losing continuity at certain interval. So the conclusion was that there was problem of coupler which connects fuel pump and ignition wires. This coupler was root cause of my car problem.
They changed coupler under warrenty and tested vehicle with my same old fuel pump and tank. There is absolute no problem after driving more than 300 kms.
So at last the problem was identified after so much trouble and it is rectified. Its all learning for them also.
I will update after 1000 kms..
I hope this laugh lasts forever while driving my car.