Tranny shifting question

My 98 Lesabre has a SERTA transmission from the dealer with 30k. When it shifts into overdrive and you take your foot off the gas to coast, when you press the gas to resume speed the tranny shifts back to 3rd and then back into overdrive. It only does it in overdrive, although it was shifting hard a few thousand miles ago. It has stopped that. What can cause this? I still have a warranty on the tranny but haven't taken it back in. Sometimes when you are going like 50 at a steady speed it will shift from 3rd to OD to 3rd to OD.

Thanks

When you take your foot off the gas, you begin to slow down. The transmission will downshift as you coast, but the shift points are at lower speeds for coasting downshifts. When you press down on the gas to speed back up, this places additional load on the transmission and it will downshift to 3rd to speed up.

In fact, if you put the pedal on the floor, the transmission should shift down to 2nd, then 3rd, then OD at higher than normal shift points (shift points occur at higher speeds due to higher engine RPMs).

At 50-55 MPH you're right on the shift point between 3rd and OD (for 25% TPS anyways). As you maintain this speed, the transmission will shift into OD. But as soon as any additional load is placed on the transmission (wind changing direction, uphill grade, etc) the transmission will downshift. At 50 MPH, even a little bit of gas will cause the transmission to downshift because you are just barely within the OD speed range.

In 50 and 55 MPH zones, I usually hold 57 MPH to keep my transmission from downshifting repeatedly. (It's just not good on the transmission and it uses gas.) If I ever get pulled over for it, I plan to tell the police officer exactly that.