One problem related to transmission shudder has been reported for the 1999 Lexus RX. The most recently reported issues are listed below.
My 1999 Rx300 awd (u140f) transmission gave 2 shudders while driving at 40 mph and then quit pulling except for 1 blk or so after restarting the engine. Took it to transmission specialist. He said planetary gears wiped out. Obtained new ones and repaired ok. 1 month later I parked and when restarted, it would not go into reverse or neutral. Had to drag out of space. Fwd ok. Drove 200 miles back to transmission shop. Planetaries again. Replaced. Ok 1 wk. Replaced whole transmission with one from highlander. Still shifts occasionally poor at 10-30 mph 2-3 shift shudder. Seems it has a maf sensor error now that may have been causing the shudder / hard shifts/failure. Car has 140 k miles. I think Toyota is hiding this simple failure causing major ($8k) failures.