My 97 does this also intermittently. However mine has been diagnosed. According to Lexus it is the IAC valve that causes this problem. $240 part plus $200.00 labor.
I had mostly run Mobil 89 octane in my ES in the last 4 months. Which by the way are my first months with car. I have been recently running Union 76 89 octane without MTBE. Have not really had the problem since although I do not believe the problem is solved just a bit postponed.