these cars use coils, like the previous mail said.
in the 16v engines they use one coil per cylinder, they are all the same and as the car is only a 4 cylinder there is obviously 4 coils.
they cost aprox 35-45 each from a local motor factor
Just a guess. Have your exhaust checked. You could have a hole somewhere close to the lamba sond. Had a problem with my renault megane which I could decribe with the same words (misfiring but only sometimes).