Most automatic transmissions when the gear selector is in DRIVE’ free wheel when the 1st gear is commanded by the Transmission Control Module. The free wheel is allowed by a one way device such as a roller clutch or sprag. This eliminates the snatch that comes as if you manually down shifted into 1st… Have you tried down shifting into manual LOW at an appropriate speed and seen engine braking while coming to a stop?
There may be a sensor error that is causing the TCM to command 1st too earily for the speed of the vehicle. When you try to accelerate again after a curve, the engine is coming up hard against the one way roller clutch or sprag giving you the snap. If this is bothersome, have a transmission specialist drive and diagnose the TCM and the power train to see if he/she can find the problem.
If the transmission specialist does observe the problem, have him/her connect a pressure gauge to the line pressure tap. It might be that the pump pressure is dropping out for some reason. Overfilling the transmission would have been an attempt to see if the pump was drawing air becuase the pick up was above the sump level. If the line pressure is dropping out, that is a case for further investigation.
Good luck on this.