2004 focus engine warning light

We recently bought a 2004 ford focus zetec 1.8 petrol. It was juddering when first started and when driving slowly in a high gear. We had a mechanic look at it who checked the spark plugs and leads. Plugs 2 and 4 where dirty, he cleaned them up swapped them all round and then started her up. Then after removing 2 and 4 they where again dirty. With what I think was oil. He then went on to change the spark plugs, leads and coil pack. But this made no difference.
When we bought the car we were told it had recently had the cambelt changed after checking this it was done very badly and hadnt been replaced properly. After fixing this issue the car ran perfectly for 3 weeks.
Then engine warning light started to flash when I was driving home one day. He came out the next day and said it had come back as a misfire on 3. And that the lead was not working anymore. He replaced the lead and it was driving fine again. That was 2 weeks ago today, driving home last night the light came on again started flashing and losing power which is exactly the same thing as last time. And it's the 3rd lead again.
We are not using this mechanic this time will use a local garage but just wondering if anyone has any ideas on why this keeps happening? As you can guess I'm not an expert when it comes to mechanics so please keep as simple as possible. Thanks