I am on my 5th rebuild now and the problem is that when I go to WOT and down shift into 1st or 2nd, I will have very little power but the rpms will go up at the speed they should and then it will shift extremely hard into the next gear.(at 3/4 throttle, everything is fine) Once I shift from 2nd into 3rd, 3rd is perfect and has full power. I am certain it is the tranny, because it has only done this the last 2 rebuilds.(fried 3-4 clutches)

 

I just realized today, that it only does this when I have driven for awhile and the motor temp is around 220. Cold, in the morning, it has full power with no slip and normal firm(not brutal) shifts thru all gears.

 

Sounds like your input sprag is slipping. Ask your trans shop if they replaced it during the rebuild. Have them check it closely because it is sometimes hard to spot the wear.