2003 gs300 hesitation

[Boo153]

Car hesitates/sucks back when pulling out from traffic light, stop sign, etc. Mechanic who doesn't do transmission repairs says it's not shifting out of high and when he used manual mode it was fine. Outside of this, car runs like a top. Transmission fluid and filters changed feb of this year. Tune up done Feb of this year. Two oxygen sensors replaced Aug of this year. Any idea what we are looking at? 

Thanks

[RX400h]

Are there any codes?