Is your Check Engine light showing any codes?
It may be your oxygen sensor going bad. The O2 sensor sends an electrical signal to the ECU, letting the computer determine exactly how much fuel it should continue to deliver to the combustion chambers. If it fails, you may get too much or too little fuel through the system. However, there are other sensors that can do this as well. Components such as the, TPS (Throttle Position Sensor), ECT (Engine Coolant Temperature Sensor) and the MAP (Manifold Absolute Pressure Sensor) sensors also send vital engine information to the computer which is used to determine proper air to fuel ratio. If any one of these components are defective they can alter the fuel delivery program. Have each sensor tested at a smog service station. They can tell you the condition of each sensor.