1997 Nissan Truck Missin/Rough Idle

I have a 1997 nissan pickup 2.4L and it has a problem idling. It hops around from 900 to 600 RPMs. If I rev it up to 1500RPM it's fine though, and it has no problems under load if you get the RPMs up. I've had the distributor, plugs and wires replaced and the fuel pump and filter are also good. The check engine light is on for the rear oxygen sensor thats behind the catalytic. Would that be causing my problems or is there something else? I'm desperate for a solution.