zafira 05 march(old model)?

my zafira just ran 29000,2 days before serviced(full) in national auto care.After that the car started to jerking on the road.especially 3rd and 4 th gear(around 30-40 mile/hr).
I returned my car to them.They told everything fine,but still got the same fault.2 times changed spark plugs.6monhs before changed the alternator as well,itscost £200.What i have to do next?Please help.