The clue is that a 60 percent increase in powder charge weight results in only a 300 psi pressure increase or only four percent. After this large increase in powder charge weight pressure remains very low with the light bullet used in the Hodgdon data. Combine this with a lightweight shallowly seated bullet and you get the wide swings in observed velocity the OP is experiencing.
If deep seating approximating wadcutter depth does not help, discontinuing its usage or using a heavier bullet is the smart thing to do. At such seating depth the light RN bullet may be barely peeking out of the case or be below the case mouth. Incrementally seating the bullet deeper is the only thing left to try with the lightweight bullet. Of course the powder charge should be reduced when doing so.
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