A few simple, easy things to check...perhaps you already have.
New plugs of the proper heat range?
Check the gap of the plugs to be as recommended ?
Have you recently untwisted (counter clockwise) the spark plug caps from the wire? Trim off a quarter inch of the spark plug cables, then tighten the caps several turns (clockwise) until they're nice and snug, and they are biting into new, fresh wire.
All these are worth a quick second look.
If none of those help...then?
Probably not carb related, since it's a single carb...if the jetting was way different from stock, you could have the symptoms you describe.
So...stock jetting? Airbox in place? Fuel pump and impulse line all good?
Probably not piston ring related, since it runs O.K. part of the time.
Now, we would have to start suspecting points/condenser (did the 79 Drifter have them, or CDI...can't remember).
Or, the external ignition coil can start to break down after it warms up.
If it does have CDI, they can begin to fail intermittently. Probably the last thing I would try...the Kawi CDI's have been pretty reliable.