I have a 2000 ES that has identical problem. All plugs were changed one year ago. My intended next steps are:
1. Change the #4 and #6 coils to see if the problem moves.
2. Change the front bank fuel rail that has injectors. (I think I am saying this correctly, but am a novice. Also, I need the posting history for the new site requirements). A buddy got one from a retailer in the salvage industry (junk yard).
3. ?

I will appreciate any helpful commentary.
Thanks.