1995 Mercedes Benz S500 car driving ten miles an hour

Engine problem
1995 Mercedes Benz S500 V8 Wheel Drive Type unknown Automatic 136000 miles

I did not have any issues with my car. Parked my car to run an errand and when I came back, my car was dragging. The car would not drive faster than five to ten miles and hour. It would alternate between going faster and slower, mostly slower.