Personaly don't load this cal. But I have had a similar situation back in the day. Wish I still had those records.....
Anyway, I would try "clays" 4.2 gr.
Where did you get your load data? Did you work it up from a smaller amount? If so, did you have the same results?
I would try a lesser amount of powder. Sounds to me as if when you fire the gun, the bullet has long since exited the barrel. The wind getting pushed back into the gun (your recoil) is pulling the powder that has not had a chance to ignite. Did you say how long your barrel was?
Try smaller amounts of powder. Or A faster burn powder.
Im at work, so I don't have my burn ratio charts on me. Someone else will have to give you a recipe on the "Faster Burn"