If you can preclude faulty pickups, cables, plug caps or plugs, then the problem has to be the amgneto itself.
As it occurs when its cold already, I could imagine faulty windings, slipring or earthing brush. Maybe the contact breaker is faulty? Could also be an out-of round armature.
If you don't know anyone to rebuild your magneto in Cyprus, I might help you out. If you are interested, drop me an email.