I know what you have been through as I went through the same with my first SC400, it would surge at idle, would even act up at highway speeds. I tried a different MAF, cleaned injectors, you name it...and no change. The Toyota shop told me originally it was the MAF and it was like $800 used....but got one off of ebay for $100. I then discovered the IACV and once I got it off found it to be the culprit....it was so dirty inside that it would not move freely, so I cleaned it up and put it back in and my Lexus purred again and was so smooth at idle....anyways this must be a secret because I don't read about it much. It is a treat to get off the engine because of the hoses that connect to it, not much give, but once you get it off of the engine and clean it and put it back in you will be happy with the results. These things are an expensive part from Lexus so it's great that it just needs cleaning. Good Luck and hope this takes care of your problems.