Car stalls and backfires into the air box only after warm up

I got a 1994 Cadillac DeVille with a Northstar
motor recently I was driving it and it just started
backfiring in the throttle body I pretty much had to
hold the pedal all the way to the floor just to get 5
miles an hour to get home after it sits for an hour
or so cools down I guess and start up and runs
fine but after about 15 minutes of warm-up it
starts stalling out and backfiring again