@Dhav you probably didn't understand my earlier message clearly, let me break it down in step by step action.
I don't doubt or believe Extreme or Munich guys but just saying for additional diagnostic because no two guys are pointing the same problem or even remotely close. However when you talk about an intermittent problem then it's genuinely a nightmare for even the competent workshop to spot the correct issue. And it's an expensive fix of either ECU or gearbox, so I would definitely go with the third opinion and then if any two diagnoses are same then I will pull the trigger.
Step 1: Find a competent BMW diagnostic center, better if they are not a workshop and just a professional diagnostic center only, so chances to get the most unbiased report. You can even check with AGMC to scan the problem if it fits in your budget - just a scan.
Step 2: Approach them saying your car switching off issue without influencing their mind with extreme or Munich diagnostic report. Let them work independently.
Step 3: Note the actual trouble codes stored in the system as per the first scan.
Step 4: Ask them to clear the code.
Step 5: If the diagnostic tool offer live data, then drive around with the diagnostic tool hooked in until some fault code pops in as soon as your engine stalls. As it's an intermittent problem so this might take a while to show up.
Step 6: If the diagnostic tool doesn't offer live data, drive around and try to reproduce the engine stalling and as soon as it happens, go back to them and ask them to scan again and get you the new fault codes stored.
Step 7: This fault code will be your actual trouble maker and then we can see if it matches with extreme or munich report to attend the right fix.
Try google and find an independent BMW Diagnostic center and get the new codes, preferably printed out of there scanner system rather than handwritten or hand type, which eliminates the room for human error or judgment.
Also check if Motrox still offering free diagnostic for Carnity members, as they had that promotion a while back.