Sounds electrical to me, something heating up. You can try swapping in known good parts, or do a little troubleshooting.
If you do not have one purchase an ignition tester that plugs in series with distributor High Tension (outout) lead. I use this one, allow you to acually see the spark when the motor is running. Should tell you if the issue is electrical or carb/fuel.
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Once it is doing it, meter the coil. Resistance across the terminals should be
about 1.5 ohms for a 6 v coil,
about 3 ohms for a 12 v coil. From each terminal to the center high tension distributor output, should be 5 - 20 k ohms. Or If you have a spare coil swap it.
Swap in a good known condenser.
Check the points gap, and ignition connections.
If fuel supply issues. Insure there is no dip downwards back towards the fire wall in the fuel line from sediment bowl to the carb. I've also heard adding a little Marvel Mystery oil to the gas can help. And have heard that clipping wooden clothes line pins to the gas line helps. Clean all gas filters - sediment bowl, carb, pencil shutoff filter.