Line Pressure/ Burnt clutches?
Sounds like a leak to me in the 3/4 area, pistons might be ok - their is an o ring in the input drum that needs replaced and oftern gets overlooked in which can cause a major leak, the input drums are also know for being cracked. Clutches might be smoked. Install a line pressure gauge and check pressure. Does it work cold and go away hot? Fluid level? Etc?
Sounds like an internal leak or burnt 3/4 clutch pack.