Head shake?

Do what the above guys suggest first then:

You should know that all bikes will headshake if the ground conditions are just right.

120km/h isn't that like 90 mph?, your not doing that on a Motocross track

If this speed is normal, and needs to be addressed, increase the front spring rate.

If this speed is above your normal riding range, but you will get there sometimes, buy a steering dampner like the Scott's or GPR. That would be a great 'quick fix'.

Also slide your fork tubes DOWN in the triple clamps as a 'race day' fix.

if you have had a new tyer fitted.... & the diameter of the wheel is now bigger...this can change the geometry of the front end of the bike & cause head shake...if you check the balance of the wheel & still have the problem ..check the headstem bearings....& if all else fails maybe look at a steering damper...

You might have thrown off a wheel weight. Go have the tire/wheel balanced again.

my 525exc had the same problem,i fitted a steering damper problem solved.it seems to be a problem with 450,525,625 ktm's