Have to say I've had / got the same problem, it only occours when driviing in a very sedate manor (Traffic jams, round town) and when the speed is just above / below the normal gear change speed (first to second gear).
It does not occour very often, I've had Lexus look at it and they found nothing wrong with the car and or gear box. If I accelerate past the normal first second gear change then I do not have a problem.