Have experienced the same problem with my 02 Sable.Have been advised that the transmission needs rebuilding or replaced to the tune of $2000-$3000.Also been told that may have the wrong type transmission fluid in the transmission.Had transmission fluid change approx. 6 months ago and Mercon transmission fluid was used.The car had experienced the problem prior to the change so I am still puzzled as to what to do.I am aware that there are different variations of Mercon transmission fluid.It could be that the the type used wasn't correct for the car.Hate to drain it and find out that the fluid wasn't the problem after all.