I was changing out the door check on the rear, driver's side, and found a white plastic part (actually, broken in two pieces); about the size of a large postage stamp, coated with grease. I think it may be a part which slides in the window regulator mechanism somehow. If anyone knows where to get it or has a part like this, apparently I need one.
Meanwhile I have a box of misc parts I have to finish using up; ordered from this site; thanks for the good service (recommended). The wood trim on this door panel has pulled loose from the aluminum backing, and is warped and curled opposite to the curve of the backing; I am trying to reglue it, but it will break I know when I force it back in place. I will have to find a replacement I guess.