After I replaced all my plugs recently after a persistent low revs misfire, old 206 was initially worse. The misfire was more frequent, especially under a light load at low revs. Then one day I was driving in normal traffic when the car gave a little surge as if someone had turned on a "go faster" switch and the misfire went away. Now the car goes like a rocket! I'm still looking for that switch, but I presume what happened was one of the COPs wasn't seating properly, as David mentions, until more driving and vibration moved it into position. I had great difficulty with cylinder 4 (I think that's no 4 - the rear one in the bank on the right hand side of the car). Very tricky getting the plug in and out and then pushing the COP back into place.
Malcolm