My (very) recently acquired '85 380SL has EXACTLY the same problem. It starts on the very first crank, but runs extremely rough. Ambient temperature is approx 5C, or 40-45F. As it warms up, it begins to smooth out. When full operating temperature is reached (15+ mins idling), the rough running is completely gone, and it runs quite smoothly. Plugs, distributor cap and rotor have just been replaced, and my new baby has been pampered from birth, with only 128,000 kms(80,000 miles). In a quite extensive search of this excellent site, there have been several conflicting answers to this type of question, in particular regarding the O2 sensor. Some suggest this is the culprit, while others say the O2 sensor has no bearing on engine performance until it is warmed up. Other possibilities center aroung the warm-up regulator, which is (for me) a very scary expensive part. Any ideas? Am I missing any input variables? thanks.