97 buick lasabre trans slips in overdrive

have you told any of the tranny places of problem when they were replacing or working on vehicle ---has the tcm been tested ---and scan for any trouble codes

Its the overdrive sensor 3rd gear you will slip with 4speed band trans
This overdrive gear setting if your sensor is not kicking in then you are due for trans maintenance as 3rd gear is shredding slowly away learned this from dirt track quarter mile super stock racing pray that your gear is intact as a trans shop with micro snake cam can only determine the length of time you have on your gears and trans inspections are free threw amco certified shops

Sounds like it's bypassing the overdrive seals. This is the solenoid package. I would put some transmission additive in first, this reconditions the seals. TranX, Lucus are both good.

I would change the fluid about 4 times,not flush.

Next replace every solenoid in the pan area

Then the filter

If it has a solenoid pack, you get all that in one piece

Do not drive it one foot, until you resolve that issue,
that comes from someone who previously owned
a trans repair business

Fluid gets changed every 30,000 miles ,not
more than 3 years on ALL Vehicles and Power
Steering at same time

It sounds like you have lost overdrive and it is slipping in third. Being a 1998, likely with a lot of miles the trans has probably burnt the direct clutches. If the ATF has a burnt smell then it's time for an overhaul.

sounds like a possible shift solenoid. If it's due for a fluid change, do that and add a container of lucas trans conditioner. Likely you have a drain plug on the pan so if the fluid was recently serviced, just drain a quart out and replace with the additive, then top off.
If the condition does not change within a week, likely you will need better diagnosis from a reputable trans shop.

no but a low fluid level could cause a slip telling the ecm there is a slip top it off then clear the code and drive a few miles if this code comes back i would say there is internal damage to the tranny

When the trans is cold it will shift almost normal as of now. If you keep driving it it will eventually slip when its cold. This is caused by worn out clutch plates
Your trans needs to be overhauled with a clutch rebuild kit

Sounds like it's on it's way out.
You will soon need to repair or replace it.
Automatic transmissions start at @$2000 & up these days, mostly up.
If you're at all handy - they are not that difficult to replace with one from a recycling yard like Pick & Pull... if you can safely get under the car.
RWD R&R is a bit easier than FWD R&R. 4WD gets a little complicated - but still doable...if you're handy with tools around cars.

Your electronic overdrive is worn... Go to a transmission specialist to have it serviced &/or repaired.