Sort of normal?
I've got a full season riding on both a Goldwing GL1500 sidecar rig, and a KLR enduro rig. Both rigs will develop head shake under certain conditions. With everything I've read so far on the various forums, and experienced personally, I believe this to be normal. On my Goldwing, I rode it first without modified steering, and then with trail reduction accomplished by installing "Steerite" modified triple clamps - what a difference! It's like adding power steering - the head shake, when it does occur, is less pronounced and easier to deal with. On my KLR rig, if I ease up or let go of the handlebars completely, under certain conditions/speeds it has headshake that will quickly develop into a tank slapper if I'm not careful. Steering is stock but quite light as is, so I just live with it. I could fit a steering damper, but am worried about it coming apart (that'd be outright dangerous!), and possibly taking some of the fun out of what has turned out to be a really enjoyable rig to go country cruising with. If you've got the rig setup correctly now so it tracks straight and steering effort is low, I'd give it some more time to get used to it. If steering effort is high, and the headshake is really something you don't want to live with, suggest you need to look into trail reduction mods.