Transmission slip

Make sure the tranny fluid level is up to spec.

If ok, do a couple of hard runs. The ECM softens the tranny's(and the engine's) performance whenever the car has been driven conservatively.

If that doesn't help, do the following:
1- Reflash the ECU by disconnecting the battery's negative cable for 30 minutes.
2- Reconnect the battery cable.
3- Turn the key to the 'ON' position so all of the dashboard lights illuminate, and the chime starts(Do not start the engine).
4- Wait for the chime to stop.
5- Turn off all electrical accessories.
6- Press the gas pedal all the way down in one smooth motion while counting to 5 (1001, 1002....).
7- Slowly release the gas pedal while counting to 5.
8- Wait 6 seconds.
9- Turn key to 'OFF'.
10- Wait 6 seconds.
11- Repeat steps 3-10 twice more.
(Note: The idle may be erratic whenever the battery is disconnected, because the ECU has to re-learn the idle strategy. 20-50 miles of driving normally returns the idle back to normal. There's also a procedure in the battery maintenance section of the owner's manual that facilitates the idle re-learning process.)

If still NG, install an aftermarket tune(assuming it's not already tuned).