Q: Takes three tries for engine to start and sluggish acceleration - 1990 Mercedes Benz 300CE

When starting the engine, I have to turn the ignition up to three times before it will start. After it has started, the acceleration is sluggish. Then after driving a while it does become a smooth ride. Also, three times within the past six months while stopped at traffic lights, the RPMs kept going down until the car quit. I replaced the spark plugs. I also thought it might be a failing oxygen sensor, but the mechanic told me that my car does not use one, but he didn’t find the problem. I really love this car and want to find the cause of this. I have been changing the oil every 3700 miles. Can you determine for me what I need to do to find my car’s problem?