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)