Don,
I think you may have revealed the problem. My recollection, though only from memory of having looked at O/U gun barrel convergence yrs ago, what that the lower barrel should 'point up' more than the upper. The rationale I came up with this was that the upper barrel has more effect on muzzle lift during recoil. I think you can check this out on a Kgun or Rem 32/3200 pretty redily since they don't have side ribs.

I'm thinking your choke bore/threads in the barrels should not be parallel. I'll check some o/u's of mine tonight.