It sounds like the transmission needs a throttle pressure adjustment. There's a piece of linkage connected from the carburetor to the transmission. It moves a spring-loaded lever on the side of the transmission that times the shifting according to throttle opening. Too much movement, and the shifts will be delayed, and VERY positive when the shift happens. You'll also get downshifts too soon, with very little change of throttle position. Too little throttle pressure, and you'll get a mushy, slippy shift. Bad juju! Adjust the length of the linkage (There's a turnbuckle adjustment in there somewhere) to get the best compromise between shift firmness and slip. Hydramatics are supposed to have a firm shift, but they shouldn't rev up high and bang into the next gear!
Jerry
