2004 Kia Sorento Won't start. Backfires through intake.

Problem started by not starting one afternoon without considerable coercing. Several minutes and several pedal stomps it started as if it were a cold untuned carbureted engine. Replaced spark plugs. Continued to have trouble starting but could be talked into starting. After a week of this, it progressed in to a shut down while driving. It's a manual, but popping the clutch while coasting would not restart. Heard loud popping when trying to start. Could smell lots of fuel out exhaust. Removed breather up to throttle body and tried to start which is when I noticed that the popping was a backfire through the intake as evidenced by fireballs in my face. CEL came on one week before all this started but both places said it was an O2 sensor. Sometimes it will start and run for 5-15 minutes with no trouble and plenty of power, but when it shuts down its out for the count.