I have recently started noticing a rough change when going from third to fourth gear on my 2002 Lasabre. I'm not sure if rough is the right description. I'ts more of like a hard bump. It is most noticeable at highway speeds. In fact I don't feel it at all around town. It has 255,000 mostly highway miles. Last fluid change was around 220,000. I'm not sure if the filter has ever been changed. No codes yet.

So what do you think guys, is the begining of the end?

Any help would be much appreciated.

That is a lot of miles on a 4t65e. There are others on here that have a harsh shifting on that model transmission and have been able to resolve it by installing the transgo shift kit. Also it is spendy but switching to dextron VI also seems to help a little.

I agree with 75 racer, a lot of miles on this transmission, maybe it'* lasted so long because they were mainly hiway miles and or good maintenance. As he said, the Transgo kit may help with your problems, but with that many miles I'm not sure what kind of shape the clutches are in even tho they are hiway miles. From what I've seen the 4t65-E is good for about 125-175k miles before rebuild, some have gotten over 200k miles and still going, but at 255k count your blessings.

2004 Bonne SLE
1971 Chevelle SS

Might be a good idea to see if any transmission codes are set...check level and condition of fluid?

Generally speaking Don to service the trans or change the fluid, a filter/ o-ring & gasket is part of normal servicing & with the kit.
So chances are if you had the fluid serviced professionally, the filter was probably replaced.
I experienced hard shifts when it got hot after hwy runs mainly and my 4t65e and would almost bang in between 2-3 gear. In the accumulator (found once you drop the pan) there are two barrels a 1-2 & 2-3 and they often can accumulate some varnish & get a bit sticky too. (same with pressure solenoids in valve body) I used 1000 grit emery cloth to smoothen out the barrels, and installed the trans go spacer & spring shift kit, which I did myself and was not that hard. It comes with good instructions. Dexron VI (6) was added and has not banged since.
When u say no codes, was it read by a professional or good code reader than can read body/trans codes? You wont get a Engine light normally.
What I would try is this. Get a can of Seafoam Trans tune: Trans Tune - Sea Foam Sales Company
Add the whole can if u got room,and see if the shifting conditions improve on highway etc. This will remove varnish from trans areas that need it including accumulator valves/barrels.
It generally takes about 500 miles of hwy driving for it to work best.
If conditions improve even slightly ..I would then consider your trans still a candidate for now a Trans go kit, and new filter/ Dex 6.
If there were ever lots of fine or chucks of particles found in the bottom of pan or on magnet, I would pass & rebuild the trans completely.
Also check your forward engine mount closest to the rad for any cracks.
I want to add for the trans to shift properly as well the engine should have a good state of tune.
Had a friend call once complaining of trans problems only to discover the engine needed plugs/wires/fuel filter Iac cleaning/throttle body cleaned, to get it to run right & shift again properly.

Thanks for all the response guys. actually the car has been well maintained but as some said " It is a lot of miles". I think it may a good idea to drop the pan and see what kind of critters may be swimming around in there. I'll definantly check out the trans tune and the transgo kit. never thought about the engine mounts.--Thanks for help everybody

UPDATE! Put in Seafoam Trans Tune. Driven about 200 miles. Problem seems to have migrated away from 3rd to 4th and moved to 1rst. When starting from stand still, I get hard bump as soon as I touch the gas. I wonder if its failing to downshift
from 4th to 1rst when I stop and then shifting when I give it gas. Strange thing is, it does it after manually shifting from reverse. I'm going to give it another couple of weeks befor doing anything else.

Check those motor mounts and transmission mount. If not that Sounds like maybe you are starting to get a Torque Converter Shudder.

I checked motor mounts earlier but not transmissiom mounts, but I will. Thanks

torqe converter shudder sounds interesting. I'm going to look into that a little more. Thanks again and stay tuned. This is not my main set of wheels so I can afford to take my time.