1994 Oldsmobile Ciera slippage and odometer

I drove my mother's car a short while back taking my mother out of town. A short quarter of the journey in the speedometer stopped working (this was the first time that I'd seen it working in recent time) and I realized that the transmission had "slipped" out of Drive and into Neutral and the engine began revving. I was quite startled and was just about to pull off the road and the transmission reengaged. It never "slipped" the remainder of our trip that day. I had checked the fluids prior to departure and they were satisfactory. The "Check Engine" Light was on , as it had been for some time. What is the problem and what is the fix for the transmission and the odometer? I would appreciate any and all insight.