04 grand marquis, suddenly shuts off
 

Hi guys, I will appreciate any assistance. I have a 2004 MGM, well kept and cared for, 2 weeks ago I was reversing from the garage, and when I put it in Drive, it died!... Nothing...however a few seconds later, ( what it seemed an eternity) I did try and started right away without any hesitation. I thought that perhaps it was the alternator, but It hasn't done it again. I am kind of "afraid" to drive it, I don't want to get stock in the middle of traffic!
I did test the alternator and it seems to be ok. the battery is new and the connectors are clean. What could it ? ANY IDEAS? THANK YOU!!!!