I have a 1990 LS400 with 125k miles. Starts fine. Runs well cold but idle seems a bit high. 800 - 900 rpm. Once it warms up the idle becomes erratic in gear or in neutral/park. It will range from 1000 rpm to 0. Eventually it dies, but will start right back up. If i hold the pedal down a bit it will stay running and the rpms stay even. Hwy mileage is 23mpg. I have replaced plugs, fuel and air filters.
Any suggestions?