My lexus ls 400 will start sometimes but only stay running for about 20 second

What is your question?

My car will start sometimes but it will only stay on for 20 seconds and then it will shut down I thought it could be the computer I change the crankshaft sensors and a camshaft sensor they all new my car is a Lexus LS400 in 1996

How long has your 1996 Lexus LS400 had this problem?

For some months