I would start simple first, a new set of plugs just to say you changed them. BTDT, they can do some strange things...
Then look at the fuel system. Filter, hose condition, pump, tank vent, etc.
Then move on the the electrical. Best time to diagnose is when the problem is present. Be ready to do the testing immediately when it fails.
If there is a fuel solenoid on the carb, be sure it is getting a good connection.
Have a spark tester ready (make one from an old plug with the electrode removed). Test the spark at each plug. Could be a coil heating up and failing, especially if it only looses one cylinder.