Suburban won't turn over

We originally started having issues with our '05
Suburban shutting off when going around sharp
curves or turns at a slow rate of speed. We knew
the power steering pump needed to be replaced
anyway, so we brought it to the shop.

They replaced the power steering pump and the
serpentine belt (it needed it), and for about 2 days it
drove fine. It began to shut off again around curves,
etc. while we were in Gatlinburg on a weekend trip.
Power steering, brakes, everything, completely
gone...but it always restarted. On our way home
from Gatlinburg, I drove across a set of railroad
tracks and the truck shut off, and this time wouldn't
restart. The shop replaced the crank sensor, with no
improvement. I am currently attempting the reset for
the alarm system, but the security light on my dash
isn't on...so I'm not even sure that would make any
difference. I have an aftermarket key, if that makes a
difference. Everything inside the truck, accessories,
lights, etc all works. The shop had it for 2 months
and finally towed it back to my house because they
can't figure it out. They has been talking to the local
chevy dealership, and they didn't know what to do
either because they said they would've done the
same as what the shop did. I'm at a loss...any
ideas?