2-3 shift slipping

Jeff, the shift from 2-3 The high clutch pack does all the work as the intermediate band releases from second gear. you may be ready for a rebuild as the clutches may be worn or leaky seals.You may be experiencing what is the torqueflight 2-3 overlap when manual shifting.Even when manual shifting at part throttle at 3500 rpm the transmission is using throttle line pressure that controls the shift.At wide open governor pressure takes over throttle pressure. You may want to play with adjusting the kickdown linkage.Not saying this is the cause but worth a try.
Basically Throttle pressure and Governor pressure are fighting each other to make the shifts.That's the best way I can explain.
Weird, I have a 727 in a truck that does the same when manual up shifting but is fine when left in D.