2000 Ford Focus acceleration

Engine Performance problem
2000 Ford Focus Two Wheel Drive Manual 86'000 miles

Hi there, ive got a ford focus 1.8 x-reg, thats done 86'000 miles... Last month I had a problem that I think it was miss fireing, as I was accelerating it would judder erratic at times and struggle to get up to speed then it would kick in again, but every time I came to pulling off it would do it again, when I took it to the garage, they said it could be the coil, but they changed the spark plugs and the lead and it was working perfect, now after 3 weeks its doing it again, can this still be the coil? The garage I took it too has now closed down, but as it was working perfect I thought that that was the problem as 1 of the spark plugs had corroded and broken down? Please help me