1990 Cutlass Supreme surges

The car is a 6 cyl, front wheel drive. It will idle rough then smooth out and back to a rough idle. When going down the highway it will surge. Not for this reason, but I have new plugs, wires, ignition module, and crank sensor on it. I think the fuel filter may need replaced, but it must be buried because I can't see it. Could you steer me in the right direction where it is at, or if I am even on the right solution? Thanks

Under the car back near the gas tank l/s of car., 3.1 right?