Sounds to me like it might be a faulty oxygen sensor or faulty heater in the sensor. On some cars the computer ignores the signals from the oxygen sensor during the first few minutes when the engine is started cold. This is because on some cars the oxygen sensor is not yet hot enough to function properly.
Maybe in your case the car runs fine when cold because the computer is ignoring the sensor. When the car warms some and the computer then recognizes the sensor, the miss returns to the engine because the sensor is sending incorrect signals.... untill the sensor gets good and hot at which time it begins to send normal signals to the computer and the engine will run fine.
But this is only one possibility. The diagnosis will take some trouble shooting. Defective temperature sending unit could also play a part.
Does not sound like cold start valve because the engine does start when cold.
[Edited by ejsharp on 04-10-2001 at 11:09 PM]
1993 190E 2.3
2000 Toyota 4x4 Tundra