Ive not seen it much with road biased tyres, but mostly with nobblies. Sometimes tyres, pressure, road surface can hit a sweet spot and the bike can weave a little. Ive one stretch on my commute that will always cause a little weave regardless of which tyre I use. I think the large 21 front makes it more noticable. Try changing variables like pressure and see if it makes any difference.

But if your still concerned. Are the tyres balanced? Are all your bearings in good order? Is the front spindle done up properly?