Did you inspect the wheel bearings themselves?
Remember you have a third bearing that carries the axle into the hub.. that bearing has it's own seal also.. I've found several on tear down that have never been serviced.
If you have a wheel bearing that is galled, it can run OK for a bit, then bind up and drag. It will make the front end pull hard.
If you are taking it down again.. have a look.. wash up the bearings and roll them while you look at the races and all the bearings.. if they are not all clean and bright.. you will see score marks on the race, and perhaps pitting on the bearings..
If every thing else checks out.. and you find bad bearings.. that's your "Ahhhhh Ha" moment..
My thinking is, you have something happening to make the hub go towards lock while you are driving.. momentary sudden wheel stoppage when a bearing locks up "might" do that.