I had this issue, they also said it was the sunlight. GM has created a software update to fix this issue. It's free and the dealer can do it in a matter of minutes. I took mine in and its running great now. Hope that helps!
2013 2LT/RS 3.6L LFX Manual 6-Speed
w/Cold Air Inductions Cold Air Intake, VMAX CNC Spiral Ported Throttle Body, 1LE Strut Tower Brace, Drake Bowtie Delete, SJM Autoprod Dash ABL kit, Side Marker Tint, Reverse Light Tint, 27% Window Tint