1998 Ford Taurus transmission problem?

Service engine light came on at about 118,000 brought it to mechanic on 2 seperate occasions because of this. Each time a sensor was replaced. The light has come on for the 3rd time now. It went out after a few days then back on and so forth. Now it has stayed on permanently. Driving to work the other day I proceeded after a stop light, car got up to 30 mph and then it felt like the car just came out of gear. The engine was revving but the car was not accelerating, just decelerating. There was no warning sign for this. I pulled over to the side of the road to a complete stop. I waited a few seconds and then just drove off. I was able to continue on without this problem occurring again but am now afraid to drive the car for fear of breaking down on the side of the road.