Twice today while I was riding I experienced this. Grabbed some front brake and they started shuddering. Don't know what could be causing this - dirty brakes? This isn't sposed to happen with a brand new bike! Might have to give them a clean and take it for a blat to see if it's helped. I read another post and if they have to be bedded in (should be done by now surely) it could just be dirt. sucks more than a vacume cleaner on how it's doing this. It only seems to do it at low speeds on downward gradients.