Usually if the modulator is shot, you get a really harsh shift. Also, if I remember correctly, you should have a mechanical linkage from the throttle linkage to the transmission. This is known as the kickdown linkage and it enables you to downshift by tromping on the gas. If this linkage is not adjusted properly, it may affect your shift point (singular beacuse it is a powerglide), in other words it may cause it to upshift early.
