Your bulbs are not as correct as you think, or you have a weak connection somewhere.

Most modern designs have a current sensor inside the flasher and if your bulbs (any signal bulb) is drawing too little current, the microcontroller inside the flasher senses it and goes into a fast blink mode. This is a safety feature to tell you one of your bulbs is potentially not doing its job showing others of your signaling. I'd consider 2000 modern.

A normal flash rate is about 1 Hz.
A sick flash rate is about 2 Hz.

Both of these signaling rates are within specifications of my local traffic law. I checked once. When was I designing a product that dealt with signaling on vehicles.

How fast is fast? If you're getting a rate much faster than 2 Hz, you have a sick system, probably a flasher fault or a wiring fault. In the old days, you'd check to see if you mistakenly put a 6 volt flasher into your 12 volt system.

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Cheers!
Scott McPhee

1987 300D