Had fuel tank changed by main fiat dealer as a result of recall on my Seicento Sporting. Nice little car before, now misfires, idling speed goes down, has stalled and judders when accelerating in low gear. Putting fuel in car has an effect, i.e. last time seemed to get better, previous time it got worse. Just had it checked by a very reluctant main dealer(same as did fuel tank), who said no problem. Now seems to be worse again and juddered badly on way home from dealer today. Help, do you think it's anything to do with fuel tank or is that red herring?