Is there a spark when it cuts out ?
2 options, either fuel or electrics.
For the fuel, check the flow rate from the tank, clean filters (air and fuel), strip and clean carb and check float bowl is not sticking.
For the electrics....not much to really check it's more a question of replacing parts and seeing if the fault disappears. Options for potential faults are, the condenser, coil, HT Lead, points and the stator itself.......plus the option of the cut-out wire shorting (but this would be more intermittent).
Good luck
