'00 70 yamaha goes foward in reverse

Just this spring my 2000 70 hp yamaha will go forward when I put it in reverse. I can go back to neutral and again in reverse and it still goes foward. If I go to foward and then back to reverse it will correct it self and may not do it again for days. Any help?

[boats.net]

Check the shift shaft, may be be rusted and about to break.