The only things I would think of would be a shift fork or a shift drum problem. They are easy fixes once you split the cases, but unfortunately splitting the cases isn't so simple and brief.
Drain the oil and see if there is metal grit or shavings in it.
You could also pull the clutch cover and take a look to see if there is anything malfunctioning in there before splitting the cases. Check the shift cam where the wheel rides on to hold it in gear. If you can get 3rd, 4th, etc. this probably isn't it, but it's worth a check.