Engine dies for no reason, idling or driving. Starts and knocks,other times starts and runs fine. 04 Silverado/Duramax 3500

This is a total head scratcher!  Sometimes it runs just fine for miles but out of the blue
it will just die.  When I try to restart it knocks really bad and I shut it right down.  After
10 minutes it MIGHT restart normally or it may not.  Next day it might start normally or
again...may not but eventually it does for no reason at all.  Certified Mechanic is
baffled.  Checked everything he could.  All seems fine.  Codes reset, parts checked
out and all seems fine.  He finally gave up.  
Truck may run normally for hundreds of miles until it does it again and other times will
only go a mile or two.  Nothing explainable triggers it.  Not heat, not potholes, nothing.  
It also blows me away that sometimes it starts back up normally and sometimes not.  
there is no rhyme or reason to it.  
Completely afraid to drive it now because the problem is so random that I never know
when its going to leave me on the side of the highway or worse yet, dying right in the
street in traffic.
PLEASE HELP with any advice or solution you may have!
Thanks in Advance!!