1994 Other Oldsmobile Models Speedometer goes crazy and car

Car starts and runs fine. Then just a few miles or less up the road, the speedometer drops to 5 or 10 mph (while the car is still going 50 - 60 mph). The transmission down shifts as if the car were going much slower. You wind up going down the road at city speeds 45 - 50 mph while the motor is spinning at 3000+ rpm. I've already replaced the transmission speed sensor. I was sure that was the problem, but not so.