I'm having a similar issue. When I come to a stop, then accelerate, the car will hesitate, then jerk into gear or down shift hard. No codes. Does not do it every time, feels like maybe throttle perhaps.

 

That could also be a solenoid in your transmission starting to go. They don't always throw codes.

 

I'm not sure what's going on. The car hasn't done the 1st gear thing for about a week now. The check engine light has not come on like it was before. I'm thinking that I'll replace the cat anyway. The guy at the shop said that the car still has the original factory exhaust. I'm just concerned that since the light has come on and given the P0420 code, that if I continue to drive the car, it could mess something else up.