Why does my car jerk uphill sometimes?

I have a 2002 Saturn SL2 (158,000 miles) and when I drive uphill, the speedometer shows the speed steadily decreasing, yet my foot is gradually hitting the pedal further and further.... then suddenly my car will give a big jerk and then the rpm goes higher to raise the speed and get me up the hill. Why does the car "jerk". Any ideas???