I may have a real simple fix for you, I have a 1992 ES with 180K on it. Back at about 158K, I had the same problem you have. I went to the Lexus dealer and they replaced the radiator cap, problem solved. From what I remember, they said this is a pressurized system and once the cap seal is compromised it causes overheating. You will want address this problem very soon with this being an aluminum engine. Hope this works for you.