Bucking and surging

What is your question?

My car bucks and surges when ever crusing and it seems to be getting worse. When there is a load on it it seems fine, the surging is very very slight. The rpm's at an idle go from 800 to 1200 to 600 to 1200 and back and forth. I'm going to try replacing the fuel filter, I don't know if it was ever done, I've had this car 3 months, it has 215,000 miles on it. Can anyone help?