I had one other thought on the cause of this. If they are blinking more from dimly lit to well lit (versus on and off). This is typically be an alternator cutting in and out. When the alternator cuts in, additional electricity flows to the bulbs, sometimes causing a notieable difference in brightness.

Sometimes this is normal, but if you are noticing an excessive difference, you should probably get your alternator/battery checked out.

Let us know whatt his turns out to be.