i had terrible wobe from mine, remove hand from bars and it would try and tank slap like crazy! In my case, it was the front tyre, carcass had shot on it, deforming the tyre.
In your case though, i'd do a few things...
1. check the bar ends are in place, remove and refit the oter way around (right onto left, left onto right)
2. since you mention top box, maybe stiffen up the pre-load on the rear a notch or 2.