RPMs jumping at highway speed

2000 neon automatic. My wife was complaining about the rpms jumping around when on the highway going about 65mph. The rpms are around 3000 and jump very slightly up and down and you can hear the engine tone change when it happens. Checked tran fluid and it was low about 3/4 of a quart and added some fluid, changed spark plugs and wires and the problem is still there. Doesn't happen all the time. She says it seems to only do this if the car has been sitting for a few hours or longer. There is an code for the downstream sensor which needs to be changed. Any ideas what the problem could be?

I was thinking maybe the TPS. I tested signal wire with throttle closed and got a .722 volts. Then with wide open throttle I had a reading of 3.76 volts. The first reading seems high and the second seems low. Would anyone else agree with me here to replace the TPS?

well the TPS wasn't the answer.....