Engine surges at idle

2000 Contour 88,000 miles, 4 cyl. automatic. Just purchased this car from an elderly gentleman that hasn't driven it in about a year. He had it inspected (PA state inpection) and serviced. One of the items replace was a sensor (O2?) near the catalytic converter. Today was the first time driving it since it was serviced. While idling, the engine will rev for e few seconds then return to normal. It cycles like this continuously. It is only noticeable at idle but may be doing it all the time. Especially annoying while stopped because car will try to lurch forward when engine surges.