My 1994 S320L doesn't get smooth idling in GEAR ONLY and happens much more frequently at night with cold weather outside .Otherwise,it is perfectly fine.When the car is idling roughly in gear, I notice the pressure gauge is fluctuating and the engine wants to go....and go...and go.Last night I noticed the transparent tube (Vaccum lines?) was very hardened.Is that supposed to be hard or flexible.Does airleakage cause that problem?? (I sent the car to an authorised Mercedes workshop and they told me the car is fine with the computer diagnosis.)This problem has annoyed me enough and it just makes me crazy!!Any help is appreciated,please!!