Have you tried adjusting the kick down cable...I know that sometimes is a problem...Just a few clicks either way can make the tranny shift much differently...I have an opposite problem of you...my tranny feels like it won't upshift into overdrive...instead of being at about 2000 rpm at 65 its racing almost near 3...I think that my Torque Converter ain't lockin up like its spose to or something like that...Maybe you might know something there...but I would say try the kickdown cable...if not have the fluid and filter changed cause there may be some dirt in the valve body....

Joe P