One problem related to automatic transmission torque converter has been reported for the 1999 Lexus RX300. The most recently reported issues are listed below.
Lexus 1999 RX 300 awd transmission failed - marked drop of acceleration power on freeway. Since original purchase of new vehicle, it has received regular service maintenance through the dealership. Last service included a flush of the transmission. Problem experienced at 74,000 miles. Dealership advised that Lexus no longer offers a transmission rebuild and that I needed to take it to a transmission repair shop (found out later that's not entirely true). Transmission has been rebuilt including solenoids, planetaries, torque converter, sun gear) at cost of over $4,200. I've since discovered that several '99 Rx300 owners have experienced transmission failure or issues at relatively low odometer mileages, and Lexus customer service has acknowledged this as well.