Corolla Transmission

I am having troubles with my transmission in a 1995 Corolla. It is the 1.8L motor. It is a three speed with overdrive. The car has 182,000 miles on it. I just recently changed out the coil, fuel filter, spark plugs and wires. The car is running fine, but it has a lot of trouble shifting. I have even tried to adjust the Throttle Position Sensor. This seems to make it better, but it does not completely get rid of the problem.

The transmission tries to shift into gear, and then it seems like it shifts right out and goes right back in. There is a real hard jerk and the motor revs up, and then it shifts into gear again. The transmission works fine if you floor the car.

I have tried just leaving the car in low gear, but it still tries to shift at about 15. I don't understand what is causing this. Does anyone have any ideas?

Thanks.