Engin ran rough or dead while at low fuel level

Hi,
 
   My 160,000 miles 1992 LS400 ran smoothly while the gas tank was above 1/3 full but when the gas level below 1/4 tank then engine suddenly surge between 400 to 2000 rpm then suddenly dead. Is there anyone can tell me what’s wrong? Thank you for helping me.
 
Raymond Jeng