The K/D band is applied in 2nd gear only, has nothing to do with 3rd or reverse. If the clutch was weak to start with the cupped orifice plug restricts the volume of fluid being sent to it. Step 1 Drop the pan and valve body. Material in the pan? If yes, pull the trans. If pan is clean. Step 2: Get a rubber tipped blow gun and feed the passage with the cupped plug with air. Clutch should thump with slight or no air hissing. Loud air hissing? Clutch piston seals or reaction shaft rings are leaking, pull trans. No material and no hiss my bet is a valvebody issue.
Doug