Before stripping the engine, check there is a quarter turn clearance on the clutch adjuster screw under the rubber plug, momentary clutch slip ? I think this adjustment may tend to close up as the clutch beds in.
Probably not a fault in the primary transmission, everything is keyed or splined and the ignition timing would go out of sync.
Sounds like she could be trying to jump out of gear, ( the highest torque is applied to the gearbox in first), so check the change mechanism, shift forks, fork guide pins, drum locator, gear dogs for wear or damage. Any other causes, damaged teeth, loose parts, broken springs, etc, should be obvious as you strip down.
Good luck, Brian, Let us know how you go on, it helps us all.