2000 Hyundai Accent Engine isn't working right

My car will crank but as it is idling the rpm will go down to about zero and the car will sputter and jerk around. The check engine light will flash when put in drive after idling and it will flash while driving and slow down and not respond to the throttle. The car's gas mileage is drastically less than it is supposed to be. There are brand new spark plugs and spark plug wires on the engine.