[korny]

If cable is intact,
First, try rocking back and forth in gears.
Just dealt with this on one of my other bikes. There it was a worn pushrod seal that resulted in a slightly bent pushrod.
Seal and rod replace, issue fixed.

 

[Rolex]

If it is a stall apply the rear brake and hold the RPM's up a bit and try again, if the clutch basket is worn the plates will have trouble separating and the extra RPM's will help.

Are you sure it is not your side stand switch cutting out the motor ?

If the switch is bad it will stop the motor every time a gear is selected, joining the two wires will bypass the switch for a test.