I've used the machine, I know how it works.
If it's a single rate spring it doesn't matter where the spring is in it's travel, the rate (and pressure) will be constant.
What you're experiencing at the beginning of the cycle is higher pressure caused by the angle of the wheel. If you start too high you can drive the wheel directly into the toe and that's when you get the chatter and even possibly a brief stop to the wheels rotation. Do you agree?