If the fluid level and band adjustments don't help,it's likely that the internal seals have hardened,causing the slow reaction when it's shifted into drive.Don't waste your time pulling the pan,since that will only remove part of the fluid.Disconnect the pressure side line from the cooler and attach some 3/8 gas hose with a clamp,and route the hose into a drain pan that will hold 2-3 gallons.Put a funnel in the filler tube(have eight quarts or so ready),start the engine and let the fluid drain into the pan,and start pouring fluid while it's draining.If the fluid is dirty,and I think you'll find that it is,you'll see clear fluid after pouring about 8-9 quarts.Shut it off,disconnect the return line on the cooler,flush the cooler with some brakleen,then blow it dry with an airhose.Reconnect the lines and take it for a test drive.This procedure gives the transmission a complete"transfusion",rather than draining the pan and replacing a portion of the fluid.I've done this for years and gotten great results.It may not completely cure it right away but it will improve gradually during the next 200-300 miles of use.There are also some very good additives that can soften and restore the internal seals,but they're much more effective with new fluid.Let us know how it works out...