things to check
- toe in
- position of brake block centre vs canti boss (too far back is good for power, bad for judder)
- brake block type (thin/hard = more judder)
- rim cleanliness
- rim deformation
- boss/pivot wear (do the arms pivot on the bosses directly or is there an integral pivot bushing in the brake arm?)
- headset free play
- cracks in steerer
Fixes for judder include
- better toe-in
- squashier brake blocks
- an uphanger
- different brake blocks (shorter and/or without an offset mounting)
- a few turns of PTFE tape on the boss in a direct pivot brake (*a)
- a few turns of PTFE tape on the boss in a brake with integral pivot bushings (*b)
- clean/dent free rims
(*a) this works by mitigating play between the brake arm and the brake boss. The PTFE tape doesn't last forever but it does last months, usually, sometimes longer.
(*b) this works (to my surprise and provided the exact right amount of tape is used) by stopping the inner part of the integral bushing moving around. One thing you shouldn't do is overtighten the bolt with this type of brake; you can easily cause the boss to fail, because the boss sees a tension load (from the bolt) with this type of brake which it does not see otherwise.
cheers