Hi Mate, no expert here..
I would have thought the old commodore would have had enough weight to counter the surge effect
Is the trailer bouncing/swaying? It could be weight distribution, not too much at the back, the hitch weight needs to be a bit not just lift with a few fingers, sometimes it's done to save the tow car's springs.
What sort of brakes are on the swift? If they are over-ride brakes (ie. the hitch bit slides and pushes the brake lever style), is the cable too tight and pulling the brakes on slightly at every bump, the cable needs to have a slight bit of slack, I would look for a small gap between the hitch nut and the brake lever.
For our 1.4t CT I don't actually feel any surging, feel the weight pushing starting/stopping or up hills but not anything I can note on cruise on the highway, mind you normally tow with a 4wd
It almost sounds like you have a really big un-baffled water tank, and it's only half full and the water is surging...
Hope it helps, best of luck with it
Paul