Have owned my brilliant 335i Coupe auto for two years, following HJ's recommendation and am as happy as Larry, then:
A month ago I was on 4-lane M-way and floored the accelerator, and at an illegal speed the engine appeared to cut out, and multiple fault messages showed. Green Flag got me back to dealer who identified an electrical short between main cable and one of the junction box terminals. Both were replaced (expensively).
This week on a single carriageway whilst overtaking a lorry, accelerated hard and the same thing occurred: Loss of drive and engine cut out. This could have been extremely dangerous!
I was able to pull in to a field entrance. The engine had stopped, but the ignition was still on. Whilst phoning the recovery service, I tried starting and the car behaved normally. A dashboard message said that I could continue my journey at moderate speeds.
The sensation when the fault occurred was that the auto transmission had jumped out of gear. Maybe then the engine had switched off to prevent it lunching itself.

Have you heard of this before with the Steptronic box. The BMW dealer has no ideas. Should I contact an automatic transmission specialist near Manchester?