Volvo: rough running at idle, fuel additive, sparkplugs

Question
earlier this year i bought a 960 (2.9lt) break with 120,000 miles.  general condition was excellent and service history was fully up to date.  4 months ago i had the volvo agent change the timing belt and make a general service.   recently, the engine began to run rough at idling speed but otherwise performs normally.  i have used an injection cleaner fuel additive and changed to a higher octane of unleaded fuel.  yesterday, i changed the sparkplugs.  the appearance of the old plugs was normal   the problem is still there with the new plugs.
according to the service records, all inspections have been accomplished.  the car has always been carefully driven.  the one previous owner is a local doctor who is known to me.     apart from the rough idling, the car drives very well with no apparant problems.   the car is a u.s. specification vehicle.

Answer
Hi Bruce,my guess is a faulty ignition coil,it is very easy to find out which one,remove the plastic cover on top of engine start the car and with the motor running unplug the coils one at time,if the idle does not change on each of the coils you have found the broken one.It is not necessary to replace them all just the broken one.