New 2015 GMC Sierra wont stay running, what could be causing this?

I just bought a 2015 GMC Sierra 1500 SLE 2wd, I purchased the demo
from the dealer with about 4600 miles, it now has 6,490 miles total.  I got in
the truck to start it and it wouldn't stay started, it would start fine, then the
tach would slowly get less and less, the truck would get a bad vibration
along with this, the tach was bouncing around and the truck would
eventually die within about 25 seconds.  No warning lights of any kind.  I
could restart the truck and the same process would repeat itself.  I probably
repeated trying to start the truck 30 or more times. I had to get it towed to
GMC, this was a Sunday.  I went to the dealer on Monday and it started
and stayed running, any ideas on what this could be?