I would disengage the shift cable at the gear shift end, go out with a friend and manually shift the gear at the tranny, then go for a motor to see if the event recurs.
That should eliminate the gear shift cable position/behaviour.. if the 'runaway' reoccurs then it's likely internal to the transmission, provided the shift lever goes properly past the detente when you shift it into gear.
Running the throttle off the engine arm (without the cable) would be another thing to try but somewhat more difficult to do.