[A.T.]

Hello Bimmer fans.
My wife's 2015 X1 is having a mysterious problem. My usually very capable mechanic seems stumped.
It's having trouble starting. It will crank but seems to be struggling for a few seconds before starting. Then it will just idle but won't rev. and will drive but only as quickly as the low revs allow. I left it to idle and warm up for several minutes. It eventually will drive seemingly normally again. But if you shut it off it will immediately have the same issue. It wasn't showing any dash warning lights, no engine light etc.
We brought (limped) it to our mechanic just a couple blocks from us. It's exhibiting all the same symptoms for him. He did a few scans and it brings up a number of random codes but different ones every time he does the scan. He's checked the battery. That appears to be good. He managed to get it running, took it for a test drive. It all looked good. He parked it. and tried to restart it and the problem was back.
If anyone has any insights it would be greatly appreciated.
Cheers
A.T.