95 Cavalier chokes at 60 mph

My wifes 95 Cavalier 2.2L automatic runs really well. Starts easy, idles smooth and gets around town with no problems. The problems start when she hits the highway. The car runs good at highway speeds (60 - 70 mph) for a few miles (5 to 10) and then starts to spit and sputter like it wants to quit. Pull off the highway to slower speeds and it runs fine again. I replaced the spark plugs and wires but that didn't help. Does this problem sound familiar to anyone, or does anyone know what might be causing this?

Get under your car and shake your catalytic converter around see if it sounds like something's loose in there. Sounds like a clogged cat in it's early stages.