Fast blinking is caused when a bulb is out. It is designed to make you go looking for the dead bulb. If it is doing it intermittently, then wait until the next time it is blinking fast, then pull over and get out. Then find the bulb that appears to be dead. Later you can pull that bulb and check the socket.
I have one of those in my wife's NB. Happens every year or so. I pull that turn signal assembly*, remove the bulb, and bend out one of the contacts at the bottom so that it has more spring pressure against the bottom of the bulb. Then the problem goes away for another year or so.
I'm thinking I need to engineer some LEDs for it, but I have too much else going on in the house and my 4-dr right now.
Cheers,
PH
* - Meaning the light bulb assembly that goes into the bumper.