I had a concept 40 on my 89 325i with the same symptoms as yours, there was a dry joint at the battery terminal , I had to crimp a new conector onto it (it looked totally fine but wasnt) this sorted it. I am sure you have looked at this already but the other thing to try ubless you are very certain ...

My guess is it Its the catalytic converter falling to bits inside.

Ive been dreading it happening to my 540i.

Hope you have a good boxing day anyway.

John.

Cool , send me a message when theres a definate date.

Am I allowed to bring my 323BHP e34 540i?