I had same problem just 3 week ago on my 2001 RX300 at 98K mile. I bought OBD scan code, $89.00 at Amazon.com with free shipping now, and scan the trouble code. I don't remember the code, however, it shows me that I have air fuel ratio A/F sensor malfunction. Ordered one from Rockauto.com, $157.00 then got local garage to replace for $40. Total cost was around $200.00. This is better than the quote from Lexus dealer $650.00. Make sure you order sensor from Denso which is Toyota / Lexus OEM, from Bosh or other will not work properly.
There are 3 sensors in RX300, 2 A/F sensor one is in front of engine and one is in back fire wall, and one O2 sensor located after manifolded. These A/F sensors are last about 100K mile.
I have some pictures or if you need more help, email me at simba2k@yahoo.com. Good luck.