Hi michelleuch,
Thank you for the donation.
Engine surging is due to a fault in the idling system, usually idle speed out of specs, (too high) and possible causes are,
1. Faulty Intake Air Conttrol valve.
2. Vacumn leakages.
3. Faulty MAP sensor.
4. Insufficient engine coolant.
5. Idle speed adjusted too high (could be A/C Fast Idle Control adjustment)
Usually a cleaning up of the throttle body and readjustment of the idle speed (if equipped with idle adjust screw) would solve the problem.