Stratovair

My 1995 e320 Coupe started idling erratically, and my dealer was unable to get any codes from the computer. They suspected the 02 sensor, so we replaced it, and no difference. The idle will lope, and when coming down from road speed, the tach will jump and then settle. The tailpie is still smelly, and I am sure it will fail another emission test. Can anyone shed any light on this problem? The car has 136,000 kilometers on it.