Hi! Yep this is definitely the sign of your steering head races being far too tight. Here's second test - stand aside the bike, both feet on the ground, and turn the bars from left to right and back a few times...do you see how the engine is thrown out to the opposite side??? When these are too tight, basically the bike is under too tight a control. Ever sat beside someone driving a motorvehicle eqiped with power steering for the first time?
Similar experience when the car veers from side to side in time with the persons breathing because they havent learnt to relax their rigor-mortis grip on the wheel lol
Problem is - once tightened down TOO far normal loose ball races mark very easy, hope you havent done too many miles like this!!! And accelerated wear sets in. So best to consider taper rollers now....cos youre gonna need them very soon.
Anyway, relax off the tightness of the races everso slightly, a few degrees of the 360degree turn of the big locknut under the top yoke. And carry the tools with you to repeat the job, until you get to the point where the bike handles fine and taut but without the side-to-side motion. Remeber to loosen the forkleg pinch bolts when you do this too, and tighten them up after, or eslse what you end up doing is trying to push up the middle of the yoke!!!
Phylo