Here's a redneck way
We made a neon sign that was a stage prop that had to flicker.
This worked for the show but doing this as a permanent sign might not be recommended by the experts.
You could hook a dimmer switch to the primary side of the transformer and reduce the voltage to the transformer.
I think the neon experts call this a flicker test
Got to be a core and coil transformer with no gfi and not an electronic.
Be careful messing with high voltage experiments.You might end up burning down someones store.
Erik from the Sign syndicate who is member here can give you the real story on this.