I was just wandering through this old thread, and noticed that one potentially important point was missing from the discussion of "chucking".
Check the tightness of the hitch ball nut. If it is even a small amount loose, the ball will snap back and forth on the mount. This will be perceived as a short sharp fore-and-aft jolt.
The big nut on the stem of the ball must have a big lock washer just above it. The nut must be tightened very tightly, mashing that lock washer absolutely flat.
Bill
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