Okbandit is probably right about it being a balance issue.
But your post reminded me of a problem I had years ago with a track bike... it too had a funny little wobble at about 120mph. I checked everything... the tire/wheel balance, fork alignment, brake rotors, run-out, it was all good. I finally realized it was aerodynamic... I changed the angle of the number plate, which was mounted to the front of the 1/4 fairing, and the wobble disappeared.
I realize if this is a street bike, you probably don't have a number plate.
But if you eliminate everything else, maybe make sure your front fender is straight and tight, and check anything else on your front end that might be causing an aerodynamic instability... just a thought.