Buick/Olds: 1999 Buick LeSabre harsh upshift, buick lesabre, upshift

Question
1999 Buick LeSabre mileage 61000.  Driven locally, all transaxle upshifts and downshifts are very smooth, hardly noticeable.  Driven on the Interstate after 25+ miles, the 3-4 upshift only becomes harsh.  Transaxle fluid and filter have been changed with no improvement.  Next action to take?  Thank you.

Answer
James

Since you say that it affects the 3rd-going-to-4th shift only, I'd have to make a strong bet on the fact that it's your Overdrive lockdown. This can be fixed relatively cheap and quickly simply by taking you car to the dealer (I know people don't like going to the dealer, but it's the only place to adjust this high-tech stuff) They will be able to very easily adjust your overdrive lockdown and it should shift smoothly.

I'm glad to hear that you've changed the transaxle fluid and filter, it is very important to  do this regularly.