So, looking for some advice, have kind of a strange situation. Here is what's up:

-Finished building up a bike with the Nashbar touring frame/fork the other day

-As soon as I take it for a spin, it's clear something is wrong. It feels like an invisible hand is shoving the front wheel to either side at intervals. It feels like a challenge to go straight and not fall, and riding no hands is out of the question.

-I've had old bikes in the past that pull to one side or the other. It doesn't feel anything like that. If you've ever done the physics experiment where you hold a spinning wheel and turn it to feel the gyroscopic forces, it feels something like that, somehow. I have three other bikes and am a pretty experienced rider so I don't think it's my riding ability that's the issue.

-I immediately suspected the headset. I had my LBS install the cups, but I did the rest of the install. I redo it a couple of times, can't find anything wrong, but it does seem like it is oddly difficult to get the headset into the zone where it's not too tight but there's no play. In fact, there's always a little play. I experimented with riding the bike with the headset obviously tight or loose but the issue doesn't go away.

-I take it to my LBS, they fiddle with the headset for five minutes and agree that it seems "off", but don't have time to investigate that day and kind of imply that a new headset will be cheaper than them investing significant time into figuring out what's up.

-The headset is a new Cane Creek 10 series.

So, I'm a bit mystified by it all. Seems like the headset is at fault simply because I can't think of anything else that could cause this, but I can't find anything obviously terribly wrong with the headset. Also the fact that the problem is there even when the headset is way too loose makes me wonder if it's something else.

Before I order a new headset slash try to return this one, has anyone experienced anything like this? Does this sound like a "bad" headset? Or like something else is the matter?

Thanks for any help...

EDIT: before anyone asks, yes I have sufficient spacers extending beyond the steerer so the headset cap can preload the bearings. I am using an expander plug rather than a star nut for the moment until I cut the steerer to its final length (though I don' t know why that should matter).