Sounds like your transmission overheated causing problems internally within the automatic transmission.
Just checking the fluid might not answer if it overheated due to a partially plugged automatic transmission filter. That too me is likely the cause if you change the transmission filter does it have metal filings?
I'd first have the transmission filter and fluid changed but have them add not one but two quarts of lucas additive then whatever particular type transmission fluid the tarus requires(it should be labeled on the transmission dipstick).
If you check the level likely it will likely say to check with the car running in park, etc...
Some transmission shops don't like lucas because it tends to swell the seals and uses other additives however its believed by others that its the polymer that tends to swell the seals in lucas that coats the bands with polymer that restores drivability when the bands themselves are the root cause of the slipping.
Personally had one car that upon overheating (fluid looked good) (previous owner changed the fluid but not the filter) shortly thereafter upon purchase the car would not move.
I changed the filter and it was full of metal fillings. I instead added two quarts of lucas transmission additive and the car has since added 34,000 miles. At this last 30,000 miles changed the filter and transmission fluids but only added 1 quart of lucas and its still running fine. A different car I tried it on didn't help thus too me it all depends what likely is happening within the transmission.
When the filter plugs it prevents fluid to cool the transmission which is why your suppose to change the fluid and filter every 30,000 miles.
If you decide to have the filter and fluid changed have the transmission shop check for metal fillings.
http://www.eliteone.com/lucas/trans.htm
Good possiblity 1-2 shift solenoids bad ?
A. A harsh shift in one gear indicates an internal transmission problem or a problem with a shift solenoid. But since the transmission was replaced, I would assume that it came with new shift solenoids. If it was just rebuilt, then it is still possible you have a bad 1-2 shift solenoid.
Also, be sure they used the correct Automatic Transmission Fluid (ATF). Starting in 1998, all Taurus and Sable vehicles require Mercon(R) V transmission fluid. 1998 Windstar vehicles built before 9/9/1997 require Mercon(R), Windstar vehicles built on 9/9/1997 and beyond use the Mercon(R) V transmission fluid.
Diagnosing GM Converter Lock-Up Problems
Edited by reversespin, : to add: possible bad 1-2 shift solenoid
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