I've often noticed that the mistake that is most often made with the 82 Pat ruck is the positioning of the waist belt.
First thing get rid of the issue belt and 'scrounge' a web belt. Run it through the frame at the back as low as you can postion it. Use tie raps or something similar to secure it in place. The function of the waist belt is to give you some relief on the shoulder straps by shifting some of the weight on to your hips. With a heavy load on a long pack I would use the waist belt shifting the weight on and off the shoulder straps.
BTW: I believe the waist belt/pack frame concept was first developed by Kelty in the sixties. Before the 64 pat, i believe that the CF had a Bergans type pack, at least the Airborne (1Can Par) did, but I'm not certain of that. Maybe someone out there knows.
Anyhow the waist belt concept 'revolutionized' back packs, Civ and Mil.