I know what you are talking about. It normally happens to me when the tank is full and the spreader is empty of vise versa, it has a slight wobble on hard surfaces.Sprayed with my Exmark for the first time yesterday. The output and droplet size is much greater than that of my old PG Magnum. The coverage appears to be good at 3/4 speed with the standard blue tips.
One thing that I am not crazy about is how the machine wants to wobble at higher speeds on concrete. Just seems to get a rhythm about it just over 3/4 speed. Anyone else see this same thing?