1994 Saturn SL1 motor surges

Engine Performance problem
1994 Saturn SL1 4 cyl Two Wheel Drive Automatic

the motor will start hard at first start, and it is just fine after warming up. Once the motor has been warmed up for about 20 min. The engine will start just fine.
while after that the motor surges (Spits and putter) at the 35 mph.
when looking down the throttle body of the carb with the motor running, revening the engine, I have noticed that as the gas in being injected with gas you can see the flow of gas shutting on and off. It will backfire once in a while doing this.