E70 50i engine off when driving, perplexed and needing suggestions

[Analyte]

Hello,
The site has been helpful before so hopefully someone has an idea. I do my own maintenance so I'm familiar with the X5 and troubleshooting things.

The wife had said her 2012 X5 50i seemed to be missing so I put the scanner on it, and I found no cylinder faults or anything else. I decided to take it down the road myself to see if I could feel what she described.

The drive was fine and I felt no issues. On the way back home at a stop sign I hit it and run it up through first to 6500 and second until around 4500. I felt no miss the entire time.

Then the engine just shut off. No warning lights, no message on the display noting a malfunction, or anything else.

I coasted to a safe shoulder, and tried to restart. It acted like the battery was almost dead slow cranking until it finally started. It ran for about 2-3 seconds and shut off again.

I pulled the scanner back out, hooked it up, and run through all the modules. No fault codes for the engine other than an O2 system error, no specific bank or anything. There was still no CEL on the dash. In addition, I now had a fault on the transfer case for oil wear.

I then tried to start it again, and another slow cranking followed by 2-3 seconds of running before shutting off. I popped the hood and looked under it as well as under the car. No obvious issues so I went back in the car to check all the codes again. Just the O2 system fault as well as the transfer case oil fault again.

I cleared them a second time, and then tried to start again. This time it cranked normally and idled normally. While it set idling I went through the scanner again, and nothing new popped up over the next 5 minutes.

I pulled back out on the road, and it was not accelerating smoothly like normal the first quarter or so. After that, it ran fine the rest of the way home. I accelerated again up the hill moderately with no issues and it was as smooth as always.

Once home I looked at everything again while it was idling. No new faults, just the O2 system and transfer case wear, and I was able to start/stop it normally a few times while trying to reproduce the previous slow crank condition.

I'm at a loss as to what happened. I'm going to investigate the O2 sensor fault a little more to see if it's a specific sensor acting up and change the transfer case fluid so those are done. Beyond that, I have no idea what caused the engine to shut off.

I guess it's possible the transfer case may have been causing a stutter or slight slipping due to the old fluid which she thought was a miss. It was just changed 30k miles or so, but I'll change it again if it says so.

Any other ideas on the cause of the engine off situation? Thanks!