Will not hold idle

I have a 1999 lexus gs300 that had over 200000 miles for some odd reason the car always cut off if I come to a complete stop unless I keep 1 foot on the gas pedal & 1 on the brake ive replace the whole engine with a rebuilt engine with 99000 miles & im still having the same issue ive replace the fuel pump as well there r time if I start the car it jus idle very high what could b the problem