I am sorry to hear that
. I should have asked what year you had before. In the book it suggests that one possiblity for a sharp
change from 1st to 2nd could be shift solenoid.
This all started from when you changed the idle, if you put it back
the way it was does it still do it? Let me know. If so I would say that that should rule out the trans.
You may want to try to also check all electical connections for corrision. Maybe the IAA valve, I am not sure though I will continue to search.