Floating Speedometer

Hi. I have a 1995 Pontiac Grand Prix, 141,000 miles with a 3.1L engine. For quite a while now, the speedometer floats. When accelerating on an on-ramp, the speedometer sometimes hangs around 40, even though I continue to accelerate, then all of a sudden will read true speed. When a computer is hooked up, the computer reads what the speedometer reads, even though the speed of the car is increasing. The speedometer also floats within 5 mph of actual speed. If I set the cruise control, the car "surges" back and forth in speed rapidly. I replaced the electronic speed sensor on the side of the transmission, but it didn't make any difference. We also tried another cruise control unit. What else could be doing this?

Thank you