I once had an issue similar to yours on my 2005 4.4 - the car would suddenly die with no warning whatsoever, even on the freeway - very dangerous as the there would be a total loss of power steering and braking would be a challenge. You better get this issue addressed and avoid driving the vehicle for now. In my case, DIS diagnostics showed code "DME- 2E95 Generator - Communications loss - Fault currently present - Fault would not cause a warning Light".

I replaced the alternator, and the issue went away. I probably could have replaced just the faulty alternator component but figured for $250 for a brand-new alternator, might just replace the whole thing and not have to worry about pulley or bearing noises coming up later.

__________________
'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.