Can you temporarily wire in a multimeter so that you can always keep an eye on the voltage at the B+ terminal (don't trust the OBD #9, it's not so accurate as a DMM) when your ignition next cuts out?

In any case, presumably your battery isn't too good as even if the alternator is intermittent, I'd expect the battery to be able to power the car.

Did you see that post a couple of days ago by shogun that required registering at a 7 series form? They described a case situation where the B+ connector itself was faulty which had the effect that the alternator wasn't always properly connected to the battery and ignition system - or something like that!