Ignition breaking up at higher rpm
Is timing trigger tight on dist shaft?
No wobble?
Is there excess up/down end play on lower dist shaft? Up-down play equals unsteady timing.
Lose a mech advance dist spring?
Flopping weights also confuse engine timing.
How long ago did it run just fine?
Corrosion inside dist cap @ plug inserts?
(Corrosion can be flaked off with a small flat blade screwdriver)