i have the same issue on a replacement trans i got from someone, they ripped me off. im fairly sure its first gear going, if i get above 17 mph and hit the gas it doesnt shudder or slip, if i *****foot it wont slip, if i gun the throttle it wont slip or shudder but launches really hard. from my research the first gear apply plate wears out and lets fluid thru so first clutches are not pressed tightly, light throttle not enought touque heavy throttle lots of pressure on the plate so it wont slip so bad, medium throttle just enough tourqe and not enough pressure to allow the slippage and get massive shudder that will shake your bowels loose
