According to this it's part #1533 for the passenger side and #1534 for the driver's side.
http://lexus.sewellparts.com/oem-catalog/1104116296-EngineAndFuel-LS400-1999.html
There is also a screen filter for the OCV's part #15846A Perhaps this has become clogged and needs cleaning.
PS: If you haven't reset the ECU by this point then you could very well be barking up the wrong tree.
Sorry for the late reply. I don't drive my IS very much. I only just turned 35K miles on it. I've had all the recalls done although I never noticed any of the symptoms Lexus mentioned. Car runs as good if not better than when it was new.
With the odometer in total mileage mode, press and hold the reset button down. While holding the button, push the car start button and watch the numbers on the odometer. It should "spin" for a bit and then show 0000. That's the reset.
Go to lexus.com and register your VIN. You will then be able to access the owners and maintenance interval manuals. I think you can also access your vehicle's maintenance history.
Yep, you just need to go drive a couple with stock suspension. 18's don't hurt the ride. The only time 17 might be ok is for snow tires. I would describe the ride as well controlled but not harsh. I think you will be pleasantly surprised compared to the other cars you mentioned.
Dealer said that doing that does not completely reset the computer. Just relaying what they told me, because I did ask that.
It does reset the ECU to default settings. Check the owners manual on smog testing.