I had a similar problem when I purchased my GS. I had probs with hesitation when accelerating from 1st to 2nd gear as if the transmission was slipping, I reset the ecu and it temporalilly fixed it. Then one day the vsc and traction control lights both came on and I lost all power when pressing the gas pedal competely to floor. Took it the dealer the next day had them check the car out and run a diagnostics. They said the tps was fine and went ahead and flushed the throttle, they aslo cleaned the air mass flow thingy out LOL. Final thought I took the car home reset the ecu one last time drove the !Removed! out the car to let it adjust to my driving patterns, I put about 20 straight miles on the car after he reset, end result no probs for the last 4 months car drives like a dream and accelerates like new. I've had no probs with vsc/traction lights coming on nor probs with lack of power or acceleration.
I hope this information is helpful and useful, cuz I was pretty annoyed by those problems abd alittle upset lexus had no idea what was wrong. There was a post here a while back referring to this topic which is where I got the idea about cleaning the mass air flow and flushing the throttle. I actually recommended that to lexus and they agreeed since my car was still under warranty. So thankx Lexus Forums. The search button does wonders!!!