My 1997 W210 E420 has about 92K miles. The other day the engine started running a little rough during idling and exhibited a little hesitation during acceleration. There was a "engine eletronics" message on the dash. This same problem occured 4 months ago and my mechanic replaced one ignition coil pack at cylinder #5 and that solved the problems. The mechanic said the other 7 coil packs were okay and did not recommend replacement. Is it possible that another ignition coil pack has gone bad and needed replacement? My other questions is how often are we supposed to replace the ignition coil packs and are they expansive?

Thanks, Garry