Drive at 55....... Just kidding.
Have you scaled your rig to see what your actual weights are? I'm no expert, but it sounds like you may have too much tongue weight and or enough bar to handle what you have.
I have an Equalizer 4 point sway control hitch, and I tow 7000 lbs with my 3500. When I first set it up, I was only interested in sway control, and not so much in weight distribution since a 3500 towing 7000 lbs is a bit of an overmatch IMHO. It worked OK for me, but I would occasionally get into the same situation you described on some stretches of undulating road in my area. I dialled in more weight distribution, and it greatly improved the tendency to porpoise.
Just my experience. Hope this helps.