New 5th wheel doesn't tow well

Lots of advice some good some bad. When we first got the 5er hooked up had some of the same problems. We put on shocks,bilstiens that helped some. lubed the hitch that helped some more. Whined about the problem to a neighbor who had a 5er about 3 years before we got ours ( 2005), he told me to shut up and change the speed about 5 miles per hour faster. No more problem. At 55 and slower it sometimes jerks back and forth, at 57 it rides as smooth as a babes bottom. At 60 she glides along, it has to do with the harmonics and the distance between the truck tires, hitch, and the trailer tires. Also include the expansion joints in the pavement and their distance from each other.Before you start spending money try a little more peddle.....