We had a guest visiting us, with a 10 year old Buick. (2001 Buick Century). He drove about two and half hours to our place, and when he got here, the transmission was shifting hard to the point that he thought that it would tear itself apart. He had a major transmission repair a couple of years ago, and he thought that it had gone again.
After Googling and reading a couple of automotive forums, I came up with a post that said to try SeaFoam (or Sea Foam) Trans Tune. I went to the automotive supply shop and bought a can of it for around $14. After adding about a cup of the stuff (half a pint), the transmission was instantly cured and shifted as smoothly as a powdered baby's bum in a velvet diaper.
I recommend this product wholeheartedly.