Lexusfixer

Check your battery terminals, if they are original, replace them. You will probably find that flickering then stops. If flickering continues have alternator checked.

I would suggest turning your key in a normal manner as you are probably causing some unintended arcing within the switches contacts, If problem does not happen under normal starting conditions consider it fixed.

You did not mention how you were driving before you came to a stop. Is this a condition that has occured after a driving on highway at highway speeds coming to the end of an exit ramp or is it a random stall condition? You may have to have diagnostic tool running while this condition occurs in which case your tech may need to drive your car until it happens again. Your second option is to seek out the most qualified tech and get the area service rep for Lexus involved.