second gear. I've also determined that itl seems more likely to happen when the car is cold rather than warm. Happened to me once yesterday on the first drive of the day, then nothing afterward, even when I got on it the car accelerated normally. It's very odd.
I'm going to try and find some time to get under the car this weekend and see if the tranny fluid is low but other than checking that I'm not sure what else to look at.