Or if your running disc brakes an out of true rotor, or maybe uneven drums or out of adjustment pads on one wheel. Does it jerk the steering or come from behind, what brake set up are you using(drum/drum, disc/drum, or disc disc? Lastly have you modified anything lately, or changed any parts? Alignment?
Just the first things that come to mind. I'd add clutch, but Allen beat me to that one.