The servo piston could be leaking pressure and not applying fully or in high gear it might not be releasing fully. In low gear, fluid is pushed against the servo to apply the band, but when you shift into high gear, the fluid applies the clutches but it also pushes on the inside of the servo to release the band.
You could have low fluid pressue due to internal leaks (sealing rings on front pump tower to clutch drum) or a weak front pump. I recommend the dual O ring servo and the Kevlar band. These items really helped the life of my powerglide.
Bill M