that's a problem with the idle air control valve or a sensor on it not the sludge build up in the engine,you need to have the computer scanned to find out whats causing the rough idle on it,or if i was you find a different dealer,sounds like the one your going to now doesn't care to much for their customers anymore,you may even need a new set of plugs in the engine now and it being a few years old.good luck.