ACC Annoyance, Limitation, Bug?

I think I've encountered this a few times:
1. Running in stop-and-go traffic with ACC active
2. Car in front completely stops and the volt completely stops.
3, Car in front starts moving and doesn't slow or stop
4. I hit "+" on Volt to get Volt moving again
5. Volt accelerates to about 6 MPH and moves about 20-30 feet and just stops for no reason.

Look in the rear view mirror and see the nice man in the Silverado about to plow me over for stopping for no reason, and then I hit "+" again and everything is fine. Be interesting to get a code/reason for why the Volt determined it needed to stop.

The other strange behavior was in the beginning of this same commute, I went to turn ACC on. Kept hitting "set" and nothing it wouldn't come on and take over. Maybe after the 3rd set attempt it finally came on and took over.

Appeared to me ACC was having a bad day - grey and overcast conditions, 50 degrees, no rain or other obstructions, flat road, not curvy or hilly.