Car cutting out, usually when I am changing gown gear.

I have the car eight weeks. Noticed the car was cutting out when I change down usually from 3rd into 2nd. Said it to the garage but they acted like it was minor problem. Warranty for 3 months only. Then a couple of weeks ago car went all shaking and shuddering (not sure buy may even have backfired). Major smell of egg. Pulled in and rang garage and thought car was finished. They got car towed back to them. After a few days they said temperature sensor was causing the car to cut out and also caused the car to shudder etc.Got car back a week ago. All fine until this morning when car, yes you guessed it, cut out three times on school runs and journey to work. Can't take it. What do I do. 3/4 weeks warranty left.