2000 Saturn L200 downshifting abruptly

I have a 2000 Saturn L200 that has over 110,000 miles on it. It drives well on the highway (over 60 mph) but as I brake to exit the highway, it often downshifts very abruptly and the service light comes on. The car does not do this every time we've been on the highway, but frequently. The light is not on when I start the car the next time, but it will come on if we've gone on the highway again. Please help!