hestitation & speedo erratic

Ist time poster. My wife has a 2000 Saturn LS 4cyl, auto, 80,000km / 50,000m. Ever since we bought it , a bought a year ago, its had a very slight hestitation at highway speed. Well, its gotten a lot worse recently. I only seem to be a problem at highway speed and now it is, as my wife calls it, "hick-upping" when it goes up hills. Normally I would think this may be a fuel filter problem, but at the same time, the speedometer is moving very erratically from the actual speed down 20 -40mph and back again. At the same time the tach does nothing unusual.
Before I start replacing a bunch of parts, any ideas????
Thanks for your help.

i would go in with a OBD-2 scantool and see what the computer thinks is the speed. the PCM does use vehicle speed in part to determine the fuel trim.