Check the channel your window moves up and down in, if it's in bad shape and is binding the window replacing that might cure it. Mostly tho it's wear and tear of the regulator itself.
http://www.67-72chevytrucks.com/vboa....php3?t=157539 has a faq on replacing the regulator. You could use it to remove the door panel and visually check the regulator for wear or tear.