i got aford fiesta zetec 1.25 16v year 2000 car, and i since i put some petrol in, the car had been shuddering laods and backfireing, i took it to a garage and they replaced the plugs and leads and its running much better, but every once in a while i get the shuddering when heavy accelerating and it shudders when i start up aswell, like its about to stall but doesnt any ideas?