Hook it up to a reader and provide us the codes.

Hopfeully not transfer case

 

I saw the exact problem these were the codes- 2869, 28B2, 27A1 and 2796: All related to SIB 12-49-06
Service bulletin 12-49-06 describes similar symptoms: "may complain about a crank/no start or rough start situation. The "Service Engine Soon Light" or the DSC light is illuminated" -Replacement of DME (computer) is required.

This led to $1800+ DME replacement, and it fixed the problem.

Here is my tip- have lot of patience and follow this:-
1. When you start the car, check the temperature gauge if its too less than half, let it idle till it dies
2. Keep a timer handy, and turn off the car (key to off position)
3. Wait for 12-14 secs after you turn off
4. Start again
5. Repeat this until you see the temperature gauge climb to about half way
6. Once you see the temperature reach half, the car could be driven until parked for longer time

Please note: this is from my personal experience and for information only. I do not assume responsibility for any problem caused due to this.

 

Do a quick search on the forum. I've seen this situation in here before with dealership saying they need a new dme, get a second opinion from another reputable BMW shop before dropping thousands