O/D light flashing -- fault code 645 -- HELP !!!

Hi. I'm looking for some insight into a tranny problem I am having. I
have a 1995 Explorer XLT 4WD Automatic V6. Had tranny "rebuilt" last
week after slipping in several gears and O/D Off light flashing. On the
drive home from shop, car is running great for 20 minutes or so, then
O/D Off light starts flashing, first gear turns to mush, and I get two
noticeable "thunks" during downshift, one at about 15 mph, then again
at 10mph. At home, using my hand held code reader, I get fault code 645
"Incorrect gear ratio-first gear". I reset EEC by disconnecting battery
and the car ran perfect for maybe 10-15 minutes, then the same symptoms
reappear. I do not know how much, if any, of the electronics was
replaced or repaired by the "mechanic", so I'm thinking that the
"repair" was incomplete, and I have lingering electronic issues that
are manifesting as mechanic problems. There are tons of posts related
to this, and I have read many, and will continue to read more. I have
heard mention of solenoids getting stuck, but don't know if this
applies to my situation. Any advise would be greatly appreciated. TIA.
- Paul

Oh yeah, forgot to mention that IF I put the tranny in the "1" or "2"
position, rather than the "D" position, first gear comes back, and the
two "thinks" disappear. The O/D Off light continues to blink, however.
Weird!?

good grief...make that two "thunks"...