Don,
My bud up here in Ak a damn good mechanic and knows his way around diesels said this..
It doesn't sound like a bad exhaust sensor. It's going into limp mode for some reason. I would find a new shop. That one sounds like a bunch of retards.
Going into limp mode was the first thing I thought of. Happens to our trucks when they idle for longer periods and don't get to run down the road and regen.
