Do you have a properly adjusted throttle pressure (kickdown linkage) installed?
Lack or misadjustment of this linkage will lead to insufficient line pressure and therefore clutch holding force, very early shift points and early transmission failure.
It's actually sad to say it but I'm not sure about the kickdown linkage. I may have just assumed on that and not done a very good inspection of things.
The shop I took it to just really dropped the pan, checked the fluid, reseated the NSS and that was about it, so no adjustments or tweaks of any kind on it.