1995 Oldsmobile Cutlass Ciera Automatic Transmission slips

1995 Oldsmobile Cutlass Ciera mileage: 94,500. This car was purchased as a dealer demo. I have changed the trans fluid and filter at least three times. The transmission fluid has a tendency to turn black quickly, but the transmission never acted up until recently. It suddenly started to lose power, so I had the transmission fluid and filter changed again. After that, the fluid once again went quickly to black, the transmission developed a hesitation while shifting, and it began to leak fluid. Now the transmission "slams" into gear somewhat when I shift into reverse or when I let off the gas pedal and then accelerate again. Any thoughts on what the problem could be or possible solutions?