Transmission question / 1999 Saab 9-5SE Automatic

On a recent ski trip up the mountains from Denver, our 9-5 SE sedan, loaded with 4 adult occupants and equipment, and set on cruise control, began to shudder and start to hunt between 3rd and 4th gears. The shudder was definately from the gear box and placing the selector in 3rd eleviated the 'hunting' however the shudder was still present. Only backing off the accelerator smoothed out the running. Flat terrain and downhills... no problem. How serious of a problem might I have? Rich Gostenik