2011 GMC Terrain stability control problems

The engine or fuel system show a severe malfunction. When at a stop light, the rpm's drop so much that it shuts down sometimes, some others, it barely stays on. I called the dealership and told me that it was not a warranty covered repair and that I had to pay for the diagnostic plus whatever was wrong to repair (parts and/or labor). All of the sudden, now the "stability trac" check light goes on an off without apparent cause. I feel like I'm driving a vehicle from the 1950's.

While driving the vehicle stalled from 35-40 MPH to 5 MPH. A message came on the screen " engine power reduced". Had I been on highway that would be ended in accident. Contacted GM and everyone I have been talking to them seems to be reading a script to me.