Had a similar problem on my 68. Even though everything looked ok, turned out to be a worn center shaft on the lower nylon roller. Not a difficult repair job if you take your time. Took out rear seat, removed quarter trim panels & watershields, etc. Degreased mechanisms, replaced all rollers, regreased, adjusted, etc. Have you replaced the quarter trim panels or weatherstripping recently? Sometimes the aftermarket stuff does not fit correctly, & causes binding. Anyway......degreasing & relubricating the mechanisms can't hurt.