I had the same problem a year ago, just when the weather turned bitter cold. At the time, my '93 had 56,000 miles.

The mechanic noted that the transmission fluid was discolored (dark brown rather than translucent red). I requested he flush the transmission (flush, not just replace the fluid). The mechanic also found a fault code pointing to a faulty sport/economy/winter switch. That was replaced.

6,000 miles and a full year later, the transmission works flawlessly and smoothly. Knowing now that transmission fluid changes do not fall under any scheduled maintenace, I will be requesting it every two to three years, and will keep an eye on the fluid color.