Having the same problem as sassee. Goes in all gears , reverse, neutrual. And sometimes in high, go about 30 feet, then slips out. Thought it was shift fork. I am going to try the emeory cloth, and see if it isnt some rust inside on the rod.

 

Check for excess play on linkage, also shifter. Adjust linkage in neutral, if still jumps out,disconnect at trans,manually shift into Hi,if still slips,probably shift fork and engagement dog wear. Also dont overlook front sprocket for damaged/ missing teeth.