I had something very similar with an RX450H that my wife had . As you say it was like driving a bus - in fact it was undriveable when it did happen . I had to switch off and start up and it would usually disappear for a short period but returned again soon afterwards . Ended up getting towed to Lexus dealers and they ran some software update or something similar , charged an hours labour and it never came back . Mind you it was very dangerous when it did happen and she had lost faith in it so we got rid of it . Shame really since was otherwise good to us . It had relatively high miles of around 120k when that happened . Hope that helps because the alternatives I was thinking of involved new steering rack at from memory around £2.5k so for once I was actually happy that dealer did not suggest that - a local independent had suggested that might be required .