I had this problem before and it turned out to be a small cast iron piece on the moter right at the end of the shift linkage. Your's could be broken or loose if you want to check it. Have someone shift the lever while you watch to see if it slips or is broken.
A real nice site to view pics of Oswego Harbor. To find your favorite marina, go to the top of the page, click on United States, then click on the State you're looking for, then the marina.
http://marinas.com/view/overview/421