To be honest, cgpro, I'm not convinced you answered the question. 3.8+ grains of W231 should be powerful enough to get almost any bullet out of the muzzle no matter where the powder is in the case. It's a light load for sure, but not so light as to stick a bullet at will.
Like Russ said, you might have a bad (contaminated, humid, whatever) batch of W231; you might have a bad batch of primers. Personally, I would look elsewhere other than powder position for the problem if you don't suspect the igniting/ignited materials themselves. I think the powder positioning is probably aggravating the real problem.
Sounds like you have eliminated the crimp - why?
Do the bullets you are using have a cannelure? What type of bullet is the 125 gr. you are experimenting with?
How large is the cylinder gap? What are the dimensions of the cylinders, especially at the mouth?
How does the firing pin strike look compared to rounds shot in another gun?
Can you try the same experiment in another gun?