I started using Arnold a lot since this was posted and had lots of problems, incl. lots of specular flickering. Ridiculous amounts of minimum AA ( 1000 ) would not stop it.
One thing I found is that if you use the Metalness feature in the shaders, your specular highlight values go way higher than if you use straight specular ( increasing IOR to 8+ gives you a similar result to Metalness )
That gets even worse if you have guys applying both metalness and specular. That’s so wrong that the Arnold guys should be ashamed of themselves for not taking care of it. It literally doubles the amount of light reflected, which is about as anti-physically-based as you can get.
If you have flickering or noise problems, definitely look at how high above 1.0 your highlight values are going. Render engines & AA can only deal with so much.
EVentually I had to drop the Spec contribution in all the lights, because there were too many shaders to fix, all of which had 100% spec intensity. Try to keep those as low as possible too.