2006 GMC Envoy code P0787 shows up.

Question

Key won't come out of ignition when wife shuts it off. Very sluggish when starting out and doesn't seem to shift thru rge gears. High RPM at 45 MPH. Sometimes it worked when it was warmer. Initially started happening on days of wxxtreme cold 10 - 25 Degree days. Any help would be appreciated. I'm handy but cash poor right now