To test if it's the headstock, you'll need two of you for this - put the bike on the side stand and lean it towards you so the front wheel comes off the ground. Get someone to hold the forks and turn them to lock position, then attempt to move them back and forth, front to back. If it's the HS, you'll feel it moving. Works for me anyway - someone of great intelligence will be along in a minute to expose me for the inept fool I am
WeeJohnyB