Jech1997LS400

Hello to all members, I have a 1997 Lexus LS400 Coach edition with 203k. I was driving on the highway doing about 70 and the car down shift and would not go over 40, so I pulled ton the side and tuned the car off waited about 15 min and turned the car on and it shifted normal, but than this happen again only this time it seem to have gotten worst. Do you know what will cause this to happen w/out any warning?