the only thing i can think of is possibly the tire itself being out of balance.
jack up the front end and spin the tire slowly if it's out of balance you'll know. the heavy spot should settle out toward the ground and rock back and forth.
they make little sticky lead weights for balancing tires and wheels. you could get some and try them probably sticking them to the inside of the wheel so you don't see them.
04' 400EX
White Bros. E2
FMF Hi Flo Headers
DynoJet Kit
K&N Filter
Hot Cams Stage1
EHS Racing Air Box Lid
DG Fat Series Nerfs
Pro Armor Swing Arm Skid Plate
Pro Taper Bars
Maxxis Razr's on Douglas Wheels
Pro Armor Foot Peg Maxi-Mizers
ASV C5 Pro Clutch Lever
LSR P-Brake block off
Modified stock carburetor
Sparks +6 key