Dealer ASAP. I'm in stop and go traffic EVERY SINGLE DAY. All I do is let the car warm up give it a few hard revvs to get that pony started and hit the road.
I had the same thing happen when I had a v6 Convertible rental in Palm Springs. I drove it a bit hard then stopped at the mall. After about 10 mins we got back in the car and every time we came to a stop it would bog and feel like it's going to stall. After we shut it off on our next stop and restarted the car, it never happened again. I never figured out the cause. When it was doing that it seemed worse in "S" mode.