Take the door panel off and check it; most likely the toothed nylon ribbon that lifts the door has stripped ( common with age )
http://www.ecklerscorvette.com/corvette-window-ribbon-repair-kit-1984-1986early.html
How to replace here
http://www.smokinvette.com/corvetteforum/showpost.php?p=133887&postcount=5
Or upgrade to the later model scissor type Window Regulators ,a bolt in upgrade
http://www.ecklerscorvette.com/corvette-door-window-regulator-left-1984-1996.html