The cause of a lot of oscillation when towing is usually the road surface. What we see as we drive is a perfectly flat road but it is not. Heavy traffic deforms the individual sections of concrete so that the roadway is actually a series of cupped sections. If your axle-hitch distance ratio and speed fall into that magic frequency, a harmonic imbalance results. This causes the trailer to jump up and the tongue to descend rapidly, causing the towing vehicle to be alternatively lifted and depressed at the hitch.
The solution is to reduce or increase speed to get the combination out of the harmonic imbalance.
It will be especially noted by towing right at the towing vehicle's limits or if over. I have experienced this with a 3/4 ton and a light single axle trailer, though. It is all about the harmonic intervals.
