Originally Posted by HillRider

This issue comes up frequently with bikes using cantilever brakes with the front brake cable housing stop located above the top race of the headset. Lennard Zinn posted a major article on this on the VeloNews web site a few months ago. His analysis was that flex in the steerer tube causes a pulsing in the brake cable that rapidly changes it's tension and, therefore the braking action.

The quickest fix was to mount a cable housing stop on the fork crown using the hole in the crown intended for a front fender or caliper brake. This, of course, only works if there is a suitable hole or you can drill one.

I don't know if his analysis is absolutely accurate but moving the cable stop to the fork crown has solved this problem for many who have tried it.