99 sable ax4n tranny touble?

[doug69]

bought car needed head gaskets repaired it seemed to run and drive great after20 min shifting from 1st to 2nd became erratic. got used trans from junkyard installed it same problem shifts like a dream for 15 miles then 1st to 2nd shifts become erratic sometimes winding up between 1 and 2 then dropping in hard replaced computer ....same! i bought a book and rebuild kit rebuilt the original trans installed it same thing?????? has new external cooler. please help. dazed and confused.

[65comet]

If you turn off the overdrive, does it still happen?