while disconnecting the transmission cooler lines you run anextreme risk of dislodging any metal chips stuck in the lines from a previous failure. This is probably how I ruined a pretty good rebuilt transmission several years back (which is the main reason I bought my parts car). When I rebuilt my current engine, I blew through the cooler lines with compressed air. Then I looked inside. There was a semi-solid plug of chips that allowed air to pass
I went nuts cleaning those metal tubes, replaced the flex lines, flushed the radiator cooler as best I could and installed a screened bolt in the transmission return line.
I sure hope it's something simple with yours, maybe a linkage issue?