I had this problem and we checked all the usual suspects.
Wheel alignment/balance,fork springs/oil,steering head bearings.
Replaced the front tyre and the problem was gone,
So I agree it'll be either a cupped tyre or steering head bearings.

Dinosaur.

"Oh Man I only ride 'em.I don't know what makes 'em work". Donald "Oddball" Sutherland

"Don't let the bastards get you down". Kris Kristofferson

"I am only paranoid because everyone is against me". Larry [Frank Burns] Linville