99 Cavalier - Intermittent Rough Idle

Hi there, I have a 99 Cavalier that will idle well, but every few seconds appears to miss, i.e. the revs dip a little. It does not spit or backfire or anything like that. I have changed the plugs and leads but within a few days the problem re-appeared. It has lots of power on the highway and good economy. It currently has 208,000 KM on it so it is high mileage, but this is the only thing wrong with it. Can anybody suggest what I can look at to change to get the idle back in check ? There are no problem lights etc. on the dash.
Thanks.

Hi there, I have a 99 Cavalier that will idle well, but every few seconds appears to miss, i.e. the revs dip a little. It does not spit or backfire or anything like that. I have changed the plugs and leads but within a few days the problem re-appeared. It has lots of power on the highway and good economy. It currently has 208,000 KM on it so it is high mileage, but this is the only thing wrong with it. Can anybody suggest what I can look at to change to get the idle back in check ? There are no problem lights etc. on the dash.
Thanks.

Could anybody tell me if the EGR valve could cause this symptom ?

Or even a PCV valve ?