I’m new here too,
I’m wondering if your radio is on when it is blinking.
Idea A:
If you have a really larger amplifier it will bring the voltage down. Adding a capacitor will help that.
Idea B:
If your radio is stock, no add-ons, it could be a weak or bleeding diode from the alternator. That will allow a small bit of alternating current to bleed back into the electrical system; when that happens the voltage will have a drop that cycles with the alternator speed. A sign of that is if the blinking is faster when you rev the engine.
So my questions are:
Is the blinking constant a speed or does it speed up with the motor?
Does the radio have to be on when this happens?