Tara, most of this problem is likely in the crank and connection rods, but some may also be due to the window itself getting a little aged.
With the window open, screen removed if necessary, remove any loose dust or debris. Then you should be able to shoot some lubricant (WD-40 or equivalent) into the back of the crank mechanism from the outside, looking in below the trim around the handle. Also lubricate the glide tracks and pivot points of the connecting rods at the top and bottom of the window. Operate the window a few times, and see if that takes care of it. If not, you'll need to look for loose screws in the jamb, signs of weathering on the window which may have caused swelling, and loose weatherstripping.