New set of Vigors, deep-section alloy, look great, good true, fast enough.
Then I put on the brakes. Awful shuddering. Dial in a bunch of toe-in, feels better but underlying problem still there. So I measure the rim width with calipers. Find that the rim is slightly flared at the places where the paired spokes go in; at these points its about .006-.008 thousandths wider than the rest. Result being that the brakes grab, release, grab, release seven or eight times per revolution. Now I see why the owners manual advises you to use an s-load of toe in.
The brake tracks are cnc machined, but this is obviously done before spokes are tightened.
I compressed the wide spots about .002, and this helps, but I am not a wheelbuilder and have no idea how far i can go. If this doesnt get fixed, the wheels will likely never see much use, so learning to live with it isnt an attractive option. I do however have a good ability to listen to professional instruction and a facility with any kind of tool you put in my hands, so I am calling Worldly Men of Experience here: what can be done, and how do you do it.
By the way, this 'flaring' thing is most prominent at the outer radius of the brake track. At the inner radius of the brake track, the rim width is pretty even all the way around.
many thanks for all comment