2001 Transmission shifts hard from 3rd to 4th

I have a 2001 SC1 that I just bought and it had the dreaded "reverse slam" and slamming from 2nd to 3rd and 3rd to 4th in the forward gears. First thing I did was "Wolfmans cure". NO CHANGE. Then I replaced was the ECTS. That resulted in NO CHANGE. I then installed a rebuilt valve body from ebay and then the transmission still slammed in reverse and the forward gears AND started to slam when it downshifted into first gear. Next I installed a new Input and Output shaft nuts and torqued them both to 112 ft/lbs. I was careful not to bend the tubes by removing the strut and fitted the cover back on by hand, all went well.
Now everything shifts great (no more reverse slam, shifts smooth from 1st to 2nd and 2nd to 3rd) EXCEPT it shifts HARD/SLAM when shifting from 3rd to 4th. It shifts fine from 3-4 when it is cold but as soon as it warms up the hard 3-4 starts. Could that be because the converter isn't locked up? I read where people messed up the tubes on the output tubes and lost 4th gear all together but I am stumped on the HARD/FIRM 3-4 upshift. It doesn't slam into 4th when I apply more throttle...it shifts HARDER/FIRMER at light acceleration. Could it be the speed sensor? Reason I ask is that it looks like the speed sensor had battery acid spilled on it at one time and it looks like the plastic is starting to corrode. Any Help would be appreciated.

It could be the speed sensor. I would replace it with a junk yard one.
Also, make sure your charging system is in check. These trannys are very picky about the voltages they see.

Ive also read that removing the battery cable and then re attaching it sometimes helps. This makes the PCM re learn your driving habits and hopefully smooth out the shifts.