Wasn't there a thread recently describing this with a software TSB? I was in a 2011 335 with DCT recently and it died after a complete stop at a light. I believe there is a fix from the dealer...do a search, but maybe Dackelone can chime in on this issue.
Found it, but not certain if this applies to you:
This is what I read somewhere:
BMW has issued a technical service bulletin for the stop / start (MSA system) issues previously reported. The problem is only related to F30 335i 3 series powered by the N55 inline six cylinder engine. The problem is that the engine will be stopped and started correctly by the stop/start MSA system, however, after the restart the engine will again shut off unexpectedly. The system will display a message to the driver that the stop/start feature has been disabled and the transmission (automatic) is put into neutral automatically.
The engine can be started manually by the driver after this has happened using the start button. This happens intermittent making this very frustrating to drivers and hard for technicians to diagnose. Furthermore the is no relevant errors logged into the ECU fault system.
BMW cites an 'unfavorable DME software application' as the source of the issue for cars produced between 11/2/2011 and 2/29/2012. To the automotive enthusiasts that loosely translates to "we screw up". BMW is owning up to the issue by issuing service bulletin 10 12 00 09 00. Everyone with a 2012 F30 3 series is of course covered under the warranty so besides the hassle of getting your 3 series recoded at the dealership it is an easy fix.