Even though I have a RC 350 (not a RX 350), I have had a similar experience. For me, it was when traction control kicked in. The motor loses some power and acts like a hesitation in acceleration. I started paying attention to the dashboard when in those scenarios that would exhibit the issue. That is when I saw the traction control light come on. I would say it was more of a 1-2 second delay before accelerating hard again.
Have you taken a look at the dash during the time the issue occurs? Have you tried to turn off traction control and see if the problems occurs? Hope this helps...